The diffusion of radical ion pairs was investigated by Monte Carlo simulations of diffusional trajectories using the drift-field approximation and an efficient algorithm for the generation of isotropic diffusional steps. The Fourier cosine and sine transforms of the conditional probability density of first reencounter, which are central quantities in the Freed-Pedersen theory for the calculation of absolute CIDNP intensities, were first obtained numerically. On the basis of these data, and by making use of the characteristic properties of the transforms, model functions were then derived, which approximate both transforms with a negligible error over the range of parameters relevant for CIDNP; six global constants suffice to calculate both sine and cosine transforms.