In this article, we analyze the impact on hotels of the ISO 14001 environmental certification system from the customers’ perspective. Based on a comparison of customer ratings of 6,850 hotels in Spain with and without ISO 14001 certification, overall guests rate the hotels with ISO 14001 certification higher than those without the certification. These results are stronger for hotel comfort and hotel services compared with other hotel attributes. Moreover, the most significant differences were found in the upscale four-star hotels. While the study does not reveal causes for these findings, the implication is that the highest end five-star luxury hotels do not gain distinctive differentiation by having the ISO 14001 certification, while for three-star hotels, guests’ price sensitivity overrides environmental concerns. At the four-star level, however, hotels seem to be able to gain a distinct market advantage from environmental certification. For all hotels, the management discipline provided by ISO 14001 can provide a competitive advantage.