Different silicas coated with poly(methyloctylsiloxane) are compared in terms of the effect of particle shape, particle diameter, mean pore diameter and specific surface area. For irregular particles, the carbon contents (% C) coated are essentially identical even though there are considerable variations in surface area for the different silica supports. Sample retentions (k) increase with increasing surface area while efficiences (N/m, naphthalene) increase as the particle diameter decreases. Excellent peak symmetries (As) are obtained with most packings. Spherical particles, with significantly lower surface areas, have lower carbon loadings and higher surface coverage but produce columns with significantly higher efficiences than the irregular particles having similar particle diameter.215645655