Theoretical relations are developed for the effect of coincidence (simultaneous arrival of two or more particles in the sensing zone) on the observed particle count in a Coulter counter, a device for counting particles in liquid suspensions. Two limiting cases are treated: (a) two or more particles are seen as a particle whose volume is the total particle volume present, (b) two or more particles are seen as the largest particle present. The theory was tested experimentally with a ``monodisperse'' polystyrene latex. It is shown that absolute particle volumes may be obtained from the ratio of solids content to number of counts under certain conditions. Case (b) coincidence predominates. The apparent breadth of the particle‐size distribution curves in the ``monodisperse'' latex was greater than expected.