An investigation of thermocapillary effects on heated menisci formed by volatile liquids in capillary pumped heat transfer devices has been conducted. This research was motivated by the importance of the evaporation process from porous or grooved media integral to the operation of capillary pumped heat transport devices such as heat pipes and capillary pumped loops. From analysis, a criteria was established which predicts the thermal conditions at which the destablizing influences of thermocapillary stresses near the contact line of a heated and evaporating meniscus cause the meniscus to become unstable. Experimentally, two different idealized models of capillary pumped phase change loops were investigated to assess the suitability of the predictions. Correspondence between theory and experiment was observed. Given the observed dry-out of the evaporator at higher heat inputs after the meniscus becomes unstable, the importance of predicting the conditions at the instability onset is made clear.