Information transparency has become one of the key subjects of environmental policies research as ethical scandals and consumer skepticism are growing. Transparency nudges in corporate social responsibility (CSR), are used to positively impact a company’s image. However, conflicting findings have emerged regarding the effectiveness of CSR messages. Further, little is known about how transparency improves consumer pro-environmental behaviour, nor whether this can backfire. Does it impact the brand and trust towards brands? Do consumers perceive it as greenwashing? Three studies (one survey and two experimental studies; N = 582) were conducted to verify if information transparency in the form of an Index of known (vs. unknown) fast fashion brands can increase pro-social purchase behaviour and trust. Findings from Study 1 suggest that the presence of an Index did not influence purchase intention and consumer evaluation of a real fast fashion brand, only the fictitious one. Study 2 highlights these findings by showing that the presence of an Index does not influence trust. Further, we present greenwashing as a boundary condition of transparency effects on trust.