Experimental data are presented showing the effects of introducing argon into the vapor space of a water heat pipe under conditions of low to moderate heat-transfer rates. At low heat transfer the presence of argon very greatly affected the thermal conductance of the heat pipe while at higher energy transfer rates the effects of the noncondensables were considerably diminished. Correlation equations are presented which take account of the effects of the presence of various quantities of noneondensable gas on heat-pipe performance.