We report the synthesis and characterization of a new vapochromic platinum compound, [Pt(CN-cyclododecyl)4][Pt(CN)4]. This compound is stable to 120 °C and reversibly sorbs three water molecules per formula unit at room temperature when it is exposed to 100% relative humidity. Dramatic changes in absorption and emission spectra are observed in response to water uptake. The vapor response characteristics of this material were further investigated by simultaneous optical reflectance and quartz crystal microbalance (QCM) measurements, using water as the test vapor. The sensing film on the QCM consisted of a mixture of the solid vapochromic powder dispersed in a hydrophobic vapor permeable polymer. The polymer was incorporated as a binder to adhere the sensing material to the QCM surface. Reflectance measurements were taken from the film on the QCM surface using a bifurcated fiber optic bundle incorporated into the flow cell. The vapor sorption characteristics of the vapochromic compound as determined by the QCM were closely correlated with the observed spectral changes. In addition, an unexpected steplike response with increasing water vapor concentration was observed, with little water vapor uptake at low concentrations, followed by a large water uptake with corresponding spectral changes, and then little change with further increases in water vapor concentration. Water vapor uptake as measured by the QCM is in agreement with the bulk gravimetric measurements. The response behavior is rapid (minutes) and reversible. The distinctly nonlinear, steplike response is interpreted as a reversible transition from a nonhydrated form of [Pt(CN-cyclododecyl)4][Pt(CN)4] to a hydrated form with three water molecules per formula unit.