On-demand ride services, such as those offered by Uber and Lyft, are transforming transportation supply and demand in many ways. As the popularity and visibility of Uber/Lyft grow, an understanding of the factors affecting the use of these services becomes more important. In this paper, we investigate the factors affecting the adoption of on-demand ride services among millennials (i.e. young adults born between 1981 and 1997), and members of the preceding Generation X (i.e. middle-aged adults born between 1965 and 1980) in California. We estimate binary logit models of the adoption of Uber/Lyft with and without the inclusion of attitudinal variables, using the California Millennials Dataset (N = 1975). The results are consistent across models: we find that highly educated, older millennials are more likely to use on-demand ride services than other groups. We also find that greater land-use mix and regional accessibility by car are associated with greater likelihood of adopting on-demand ride services. Respondents who report higher numbers of long-distance business trips and have a higher share of long-distance trips made by plane are also more likely to have used these services, as are frequent users of smartphone transportation-related apps, and those who have previously used taxi and carsharing services. Among various attitudinal factors that were investigated, individuals with stronger pro-environmental, technology-embracing, and variety-seeking attitudes are more inclined to use ridehailing. These findings provide a starting point for efforts to forecast the adoption of on-demand services and their impacts on overall travel patterns across various regions and sociodemographics.