To what extent are epistemic curiosity and situational interest different indicators for the same underlying psychological mechanism? To answer this question, we conducted two studies. In Study 1, we administered measures of epistemic curiosity and situational interest to 158 students from an all-boys secondary school. The data were analyzed using confirmatory factor analysis to find out whether a one-factor or a two-factor solution provides the best fit to the data. The findings supported a one-factor solution. A two-factor solution was only satisfactorily supported if one accepted that the two latent constructs were correlated .99. Study 2 was an experiment in which we experimentally manipulated the amount of prior knowledge 148 students had about a particular thermodynamic phenomenon. Epistemic curiosity and situational interest were each measured four times: before a text was studied, before and after a problem was presented, and after a second text was read. The treatment group studied a text explaining the problem "after" the problem was presented, whereas the control group read it before the problem was presented. The control group, in other words, gained prior knowledge about the problem. In the treatment group, both epistemic curiosity and situational interest significantly increased while being confronted with the problem. This was not the case in the control group. In addition, only in the treatment group scores on both measures significantly decreased after the text explaining the problem was studied. These findings support a knowledge gap account of both situational interest and epistemic curiosity, suggesting an identical underlying psychological mechanism.