The fact that the incoherent backscatter spectrum narrows when the radar beam is nearly perpendicular to the magnetic field is well known and has been used at Jicamarca for more than 30 years to measure very accurate line-of-sight velocities. Recently it has become clear that these spectra are narrower than expected. We have explained this effect and also the small change to the spectral shape required at somewhat larger angles to correct the ratio of electron to ion temperature seen in some studies. Coulomb collisions affecting the motion of the electrons are responsible for the additional spectral narrowing. We have carried out very accurate simulations of electron motion resulting in incoherent scatter spectra which are qualitatively similar to spectra resulting from other types of collisions, and to those predicted in an analytic solution for the Coulomb case [Woodman, 1967>. However, we found that the spectrum of the velocity time series in the radar line of sight departs significantly from the nearly Lorentzian form expected with simple collisional models. This causes the collisional effects to extend to somewhat shorter scale lengths, or further from perpendicular to the magnetic field than expected. In order to investigate the collisional process more closely, we performed another simulation combining the effects of electron-ion collisions and a simple friction model (Langevin equation) in an adjustable combination. This one showed that the effect of electron-ion collisions alone would result in collisional effects extending several degrees farther from perpendicular to the field than when both kinds of collisions are included. Collisions affecting the speed of the electrons tend to limit the size of the effect at larger angles from perpendicular. Thus the effect of these collisions on the incoherent scatter spectrum cannot be accurately predicted from simple models but depends on the detailed physics of the collisions. ¿ 1999 American Geophysical Union