This paper details the development and application of a novel hydration apparatus, used to initiate water-mediated reactions in an isothermal microcalorimeter. Conventionally, the relative humidity (RH) in a sample ampoule is maintained using one of two methods; an amount of a saturated salt solution is held in a small glass tube (Durham tube or mini-hydrostat) sealed within the ampoule or a carrier gas of known RH is flowed over the sample at a constant rate. The Durham tube method is often used on the basis of simplicity and cost, but has several disadvantages; the system may be rate-limited by the small surface area available for water evaporation, the internal surfaces of the ampoule are wetted and the system cannot reach equilibrium before the reaction is initiated. In this work, we show how by switching the placement of the sample and water reservoir (sample in a sealed hydrostat and water reservoir in the ampoule) many of these drawbacks are overcome. The use of the system is demonstrated by studying the hydration of anhydrous ceftazidime.