This paper investigates the performance of transmitter power control algorithms in a physical layer network coding (PNC) system.  Both the optimal, co-channel dependent (CCD), and suboptimal co-channel independent (CCI) power control schemes are considered in terms of their effect on the capacity of the multiple access stage of the two-way relay channel.  It is found that for a flat Rayleigh fading channel, and a desired channel capacity, the use of the CCD algorithm provides an average transmitter power savings of approximately 2dB relative to not using transmitter power control.  The CCI scheme based on single user water-filling is found to perform worse than using no power control at all.