Water vapour absorption (WVA) is an important mechanism for water gain in several xeric insects. Theoretical calculations indicate that the energetic cost of WVA should be small (5–10% of standard metabolic rate) assuming realistic efficiencies. In this study we explored the relationship between WVA, metabolic heat flux (HF met.) and CO 2 release in larvae of Tenebrio molitor using microcalorimetry. By comparing metabolic heat flux with the catabolic rate estimated from V CO 2 , we were able to differentiate anabolic and catabolic rates prior to and during WVA, while simultaneously monitoring water exchange. Three to four hours before the onset of WVA, larvae showed clear increases in HF met. and catabolic flux, and a simultaneous decrease in anabolic flux. Following the onset of WVA, HF met. decreased again until indistinguishable from control (non-absorbing) values. Possible factors contributing to the “preparatory phase” are discussed, including mobilization of Malpighian tubule transporters and muscular activity in the rectum. Absorbing larvae reduced the water activity of the calorimetric cell to 0.906, agreeing with gravimetric estimates of the critical equilibrium activity. Periods of movement during WVA coincided with decreased uptake fluxes, consistent with the animal's hydrostatic skeleton and the need to close the anus to generate pressure increases in the haemocoel.