Homogeneous nucleation rates as functions of both supersaturation and temperature were measured in an upward thermal diffusion cloud chamber for four n-alkanes : n-heptane, n-octane, n-nonane, and n-decane. Nucleation rates from about 10(-4) to 5x10(0) drops cm(-3) s(-1) were obtained in the temperature range, 241 to 330 K. Their dependences on supersaturation and temperature were compared to predictions of several nucleation theories : the internally consistent Classical theory, two versions of the Kalikmanov-van Dongen theory, and the Delale-Meier theory. Each theory predicted the dependence of the nucleation rate on supersaturation reasonably well. However, large temperature dependent correction factors were needed for quantitative agreement between measured and predicted nucleation rates. A plot of the ratio of measured to predicted nucleation rates vs reduced temperature shows that all n-alkanes investigated can be represented to within a factor often by a single, best fit line.