Interparticle spacing (IPS) is a very important parameter for estimation of the viscosity of a suspension. A new equation for calculating IPS was derived on the assumption that each particle is Surrounded by a Virtual cell, which is the free volume that each particle can occupy. This idea was originated by Kuwabara and widely used for calculating electrophoretic mobility in concentrated suspensions. Our new IPS equation was evaluated and compared with a pre-existing IPS equation proposed by Dinger and Funk using experimental data from three commercial sources of titania. Our equation was found to give reasonable values, including cases where the equation of Dinger and Funk gave anomalous values. (c) 2006 Published by Elsevier Inc.