An experimental and theoretical investigation of basic plasma-physics processes relating to the absorption of electromagnetic waves in collisional plasmas is described. One-way absorption of 63 dB at 4 GHz was demonstrated in a section of plasma-loaded rectangular C-band waveguide. The ultraviolet photoionization plasma-production technique employed permits independent variation of the plasma-density profile and electron-collision frequency. A theoretical model for the absorption and scattering processes, which includes scattering contributions from the plasma-vacuum interface, and partial reflections and collisional absorption in the bulk plasma, is in reasonable agreement with the experimental results.