The concept of social capital has become very popular but its definition and measurement are still rather unclear. We frame our study in one of its components, social participation. In this article we develop an optimal measure for social participation based on the questions asked in the first round of the European Social Survey. Our analyses suggest that a distinction has to be made between informal and formal participation because they relate very differently to other variables such as age, education, political action and happiness. We also found that these two types of participation had hardly any relationship with other important components of the social capital construct, such as social trust and political trust. The latter result does not devalue the validity of the developed indices for informal and formal participation but suggests that participation and trust should be considered formative indicators of social capital.