In this paper we show two original alternative models. Model 1 involves two sources for the adoption of environmental practices (PRAC): the collaboration-oriented human resources management system and the organizational learning capability. Model 2 adopts three sources for PRAC: the collaboration-oriented human resources management system, the organizational learning capability and information technology support (ITS). Both models also use three moderating variables: the manager's education level and experience, and the firm's size. We use a mixed-methods approach with structural equation modeling and a fuzzy-set qualitative comparative analysis on 349 small and medium-sized Portuguese firms. The results show that collaboration oriented human resources system, organizational learning capability, and ITS have a positive effect on PRAC. In addition, the individual characteristics of the managers significantly improve the explanatory power of the models. However, the firm's size has a negative moderating effect. Therefore, the smaller the firms are, then the larger the positive effects of the antecedents on PRAC. The results also show that ITS is an important source for the development of PRAC, and it contributes to different pathways that lead to PRAC. But, no alternative causal configurations exist that lead to PRAC regarding Model 1, yet alternative causal configurations do exist that lead to PRAC regarding Model 2. In this line, the findings from both the quantitative and the qualitative analyses show the relevancy of ITS to PRAC.