Cross sections for the (p, pxn) reactions of Br79 and Br81 have been measured at 2.9 GeV. The cross sections decrease monotonically as x increases, and the decrease with increasing x is faster for Br79 than for Br81. The results are compared with Monte Carlo cascade-evaporation calculations. The experimental values are larger than the calculated ones for x<=3 but are in good agreement for x=4-5. The systematics of (p, pxn) reactions at high energies are considered for a number of targets. A quantitative analysis of the (p, p3n) and (p, p4n) reactions indicates that the evaporation process determines the variation in cross section from target to target.