The contact angle of ultrafine size particles has been evaluated using 1 mu m monosize SiO2 particles of various degrees of wettability. The contact angle was determined by film flotation and Zisman plots. Chlorotrimethylsilane (CTS) was used to methylate the SiO2 particle surface and establish the level of surface wettability. Also, the vacuum floatability of the methylated ultrafine SiO2 particles was assessed to correlate it to the contact angle. This vacuum floatability was very low below 40o and increased monotonically above this contact angle value because of favorable bubble nucleation and a greater stability of the bubbles on the hydrophobic surface. Free energy of bubble nucleation on the hydrophobic surfaces has been estimated and correlated to the vacuum floatability of the ultrafine particles.