The objective of this article was to analyze the factors that influence micro, small and medium-sized Mexican companies in the adoption of e-commerce. Data from the 2019 Economic Census, conducted by the National Institute of Statistics and Geography, were used. The sample included 135 775 companies that reported online revenues in 2019. For the analysis of the information, Tornatzky and Fleischer's technology, organization and environment framework was taken as a basis, and a multiple linear regression model by firm size was created. The results reveal that firm size, cash sales, having a website and intranet access have a negative relationship for e-commerce adoption, while access to and use of e-mail, computers and the Internet have a positive relationship. This is because technological integration becomes less important as the company grows, going from being a facilitator to an inhibitor of the acceptance of online sales. This paper demonstrates that technological, organizational and environmental characteristics have significant effects on the adoption of e-commerce by MSMEs in Mexico; the findings may be of help to owners and managers, as well as to policy makers in Mexico responsible for expanding the number of firms adopting e-commerce.