The authors used a Wilson expansion cloud chamber to measure binary homogeneous nucleation rates for pure n-octane, pure i-octane, and 3:1, 1:1, 1:3 (mole ratio) mixtures in a temperature range from 215 to 260 K. The nucleation rates range from approximately 100 to 50 000 drops/cm(3) s. Current binary nucleation theory is unable to predict the data for this nearly ideal system.