In the first of two papers studying diffusion in bimodal and polydisperse polymer solutions, the self-diffusion coefficients of various protonated polymers in a series of bimodal solutions containing deuterated polymers have been measured. Utilizing the chemical specificity of the pulsed-field gradient nuclear magnetic resonance (p.f.g.n.m.r.) technique, it was possible to study the effects of both molecular weight and concentration of each component. A simple friction dependence was shown to account for the observed trends. The p.f.g.n.m.r. results are contrasted with mutual diffusion coefficient measurements, obtained from multi-tau photon correlation spectroscopy on similar bimodal systems.