The critical supersaturations and isothermal homogeneous nucleation rates of dodecane, hexadecane, and octadecane vapors have been measured over wide temperature ranges (e.g., 285-340 K) using an upward thermal diffusion cloud chamber. Dodecane shows the best agreement between the experiment and the classical nucleation theory. Hexadecane and octadecane exhibit critical supersaturations lower than those predicted by the theory. All three compounds show increasing deviation from theory with decreasing temperature. This trend is also observed in the measured nucleation rates where the predictions of the theory at lower temperatures are shifted to much lower values compared to the experimental data. Temperature dependence correction factors to the calculated rates have been evaluated based on the experimental rates of the studied higher alkanes. The classical theory predicts the correct supersaturation dependence of the nucleation rates of these compounds. Analysis of the supersaturation dependence of the nucleation rate of octadecane indicates that the critical cluster contains 30-35 molecules at the nucleation temperatures of 315-340 K. The classical theory predicts smaller nucleus size as a result of the overestimation of the supersaturation corresponding to the measured nucleation rate.