The Pioneer 10 and 11 missions to Jupiter and beyond have provided the first opportunity to observe solar flare acclerated particles at distances of several astronomical units from the sun. The intensity-time profiles out to at least 6 AU ae consistent with mainly diffusive interplanetary propagation. No obvious effects of a free escape boundary have been observed, which implies that the outer boundary of the diffusion region is beyond to AU. Four solar particle events are discussed in detail. These are events during which azimuthal propagation and corotation effects played a relatively minor role at least until the time of maximum intensity. Their intensity-time profiles, which were observed simultaneously at two or three well-separated radial positions (with Imp 6 or Imp 8 at I AU and with Pioneer 10 and/or Pioneer 11 at 2.7-6.1 AU), are used to study the radial transport of energetic particles in interplanetary space. The events are interpreted in terms of a spherically symmetric propagation model (which is also appropriate within the flux tube from the coronal release site even if K∥≫K⊥) which includes the effects of diffusion, convection, and adiabatic deceleration and which assumes that the particles are impulsively injected at the sun. If the effective radial diffusion coefficient is assumed to have the form Kr=Kr(1 AU)(r/re)b, the average value of the radial index b is found to be 0.4¿0.2. The average radial diffusion coefficient at 1 AU for 11- to 20MeV protons is Kr(1 AU) = (1.7¿0.2) ¿1021 cm2/s; for 30- to 67-MeV protons, Kr(1 AU) = (2.9¿0,2) ¿1021 cm2/s; and for 1- to 2-MeV electrons, Kr(1 AU) = (4--9) ¿1021 cm2/s; all for the period 1972-1974. The corresponding radial mean free paths at 1 AU are &Lgr;r(1 AU) =0.065¿0.010 AU independent of energy for protons from 11 to 67 MeV and &Lgr;r(1 AU) =0.03-0.06 AU for the 1- to 2-MeV electrons. The measured solar wind speeds are used to estimate the solar connection longitude of each spacecraft in order to interpret better the observed intensity-time profiles in terms of radial propagation and corotation. It is found that when the calculated solar connection longitude remains nearly constant during an event (a 'dwell' region), a period during which corotation effects are minimized, the model intensity-time prediction tends to agree with that observed even in the decay phase of the event. The maximum intensity is found to decrease with radial distance as r-3 to r-4, which is consistent with the cross-sectional area of the flux tube containing the particles increasing as r2 (or somewhat faster than r2 in dwell regions).