A number of recent formal models predict a positive effect of political knowledge on turnout. Both information acquisition and turnout, however, are likely to be determined by a similar set of variables, rendering hard the identification of a causal link in empirical investigations. Available empirical regularities should therefore be interpreted as mere correlations. I address this problem by using an instrumental variables approach, where the instruments are represented by various proxies of information supply on mass media. Using survey data from the 1997 British General Election Study, I show that political knowledge has a sizeable influence on the probability of voting and that mass media play an important role in influencing political participation.