We have measured and calculated doubly differential single-ionization cross sections for 50-150-keV p + He collisions as a function of the projectile energy loss and scattering angle. The measured cross sections were put on an absolute scale by normalizing the integrated cross sections to known total cross sections. Clear effects of the postcollision interaction (PCI) were observed in the scattered projectile spectra. Our singly differential cross sections as a function of projectile energy loss do not agree with corresponding cross sections obtained from electron spectroscopy. These discrepancies are attributed to the PCI. Our calculations including the PCI reproduce the shape of our data quite well; however, there are discrepancies in the absolute magnitude.