A statistical formulation of ion-neutral interactions in rf Paul traps is presented which provides a basis for calculating the collisional relaxation of a trapped ion cloud. These calculations describe the ion cloud as a statistical distribution in space and energy and treat collisions by Monte Carlo methods. Ion motion is calculated in the presence of space charge fields in a mean field approximation which allows each ion to be treated independently. This formulation provides an adequate description of the ion cloud dynamics for ion densities less than or equal to 10(8) cm(-3). Frequency shifts induced in the ion cloud ensemble by space charge are characterized. Energy relaxation of a C-60(+) ion cloud initially at a temperature of 5000 K by collisions with a background He gas at 300 K is calculated in the presence of space charge.