We generalize the second nucleation theorem to multicomponent systems. Nucleation theorems are used to extract the molecular composition and excess internal energy of the critical cluster from experimental nucleation rates in a water-ethanol mixture. The excess internal energy is found to depend only weakly on temperature and to be almost solely a function of the molecular numbers of water and ethanol in the cluster. We estimate the contribution of the kinetic pre-factor to our analysis, and find that it is small in the case of the first theorem, but significant for the second theorem. We find that capillarity approximation fails to predict the experimental critical size and excess energy in this highly nonideal system.