We investigated the solvent vapor annealing process of thermally evaporated thin films of the [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl) spin crossover complex. Using grazing incidence X-ray diffraction, atomic force microscopy, and variable-temperature UV spectrophotometry, we show how the crystallinity, morphology, and spin crossover properties of the resulting films are altered by exposure to vapors of five different solvents: diethyl ether, acetone, dichloromethane, ethanol, and water. Remarkably, most solvents yielded highly oriented crystalline films displaying an abrupt and complete spin transition around 336 K. A notable exception is the case of dichloromethane—suggesting that the solvent ability to accept hydrogen bonding might be a key factor in controlling the film crystallinity. On the other hand, only water vapor treatment affords for smooth and continuous film morphology. The recrystallization process in controlled humidity was thus followed in situ by monitoring the UV absorbance of the films. This study revealed the critical importance of the relative humidity (RH): for values above a threshold of ca. 72% RH (at 298 K) a very fast (seconds) crystallization occurs leading to robust, crystalline films, whereas crystallization at lower RH values is slower (hours) and leads to less stable, semicrystalline films displaying incomplete spin transitions.