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Although the association between sustainability assurance (SA) quality and firm value has been examined in previous studies, the moderating relationship is novel in this study and highlights the effect of corporate environmental sustainability performance (CESP) on the relationship between SA quality and firm value. This study aims to examine whether such an effect is strengthened or weakened by eco-efficiency, as measured by ISO 14001 certification, aggregate CESP score and each individual dimension of CESP (emission reduction [ER], resource reduction [RR] and product innovation [PI]).

The sample includes 40 companies in Euronext Paris with the largest market capitalisations (the Cotation Assistée en Continu 40 [CAC 40] index) from 2010 to 2020. The authors apply the feasible generalised least squares regression technique to estimate all the regression models. Because observed associations may be biased by reverse causation or self-selection, the authors use the instrumental variable approach and Heckman two-stage estimation.

The results show that SA quality had a positive and significant effect on firm value. Second, the authors demonstrate that CESP, as assessed by ISO 14001 certification, has a stronger interaction with assurance quality and acting as a moderator variable. Using the ASSET4 scores, an alternative proxy for CESP, the authors find inconsistent evidence regarding the impact of CESP attributes. The CESP and ER scores are homogeneous and have a positive effect on firm value. However, the PI and RR CESP attributes are not homogenous and do not have the same interactive effect on firm value. The results are robust to the use of an instrumental variable approach and the Heckman two-stage estimation procedure.

Policy implications: Regulators may be interested in the findings when considering current and future assurance requirements for sustainability reporting, and shareholders when considering SA as an investment choice criterion. The insights into and enhanced understanding of the incentives for obtaining high SA quality can help policymakers develop effective policies and initiatives for SA. Considering the possible improvements in sustainability performance when obtaining a high level of sustainability verification, governments need to consider mandating SA.

Firms receive clear confirmation of the importance of investing in SA quality. Financial markets do not evaluate SA dichotomously but reward companies with higher SA quality because of the greater credibility it provides. Firms should allocate a significant percentage of their annual budgets and other relevant resources to environmental training and development programmes to improve and maintain environmental performance. If they care about environmental issues, they must announce this by issuing sustainability reports and seeking assurance of the information disclosed. High-quality assurance not only has a significant effect on investors’ investment reliability judgements but also the perceived credibility of environmental performance fully moderates the effect of assurance on these judgements.

This study has social implications; the authors find that the French market rewards firms that provide a high-quality assurance to guarantee the integrity of their sustainability reports. Therefore, by incorporating environmental sustainability into their financial goals, a better assurance ultimately will urge firms to move from green washing to strategic goals, which is beneficial for society. Further, firms that focus on sustainability as part of their business strategy may attract employees who engage in green behaviours at work and create a friendlier and productive environment because it gives meaning to the work they do and keeps them engaged to the level needed to perform their jobs capably.

This study contributes to the literature by re-examining the relationship between SA quality and firm value. It also provides new evidence on the moderating effect of CESP on the SA quality–firm value nexus. Specifically, it explores the joint effect of credibility and eco-efficiency on market confidence in sustainability information.