In the search for ways to encourage more participation in voluntary activities, more attention is being given to what motivates people to participate. Participation in voluntary activity is usually explained by rational choice theory, but theoretical predictions have not always concurred with empirical outcomes. We proposed a theory of volunteer opportunity and verified the theory by using a web survey. Our central hypothesis was that having access to opportunities leads to higher participation rates in voluntary activity. Our analysis of the factors involved in participation in voluntary activity supported the hypotheses based on this theory. For example, number of interests, some sources of information related to the area and specific activities, and other factors connected with participation opportunities had significant positive effects on participation. Factors related to egoistic motives, however, were either not significant or had significant negative effects, and economic models were thus not supported. It was clear that participation in voluntary activity is related to number of participation opportunities, and we suggest further development of the theory of volunteer opportunity.