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We report on fast sensing of air humidity using the leaky surface acoustic wave (LSAW) propagation in YX-LiTaO3 with hematoporphyrin (Hp) nanostructured film. Effect of humidity on transmission characteristics of the Hp-LiTaO3 two-port SAW delay line was investigated using RF network analyzer. Both the amplitude and phase of transmitted signal become strongly dependent on humidity if the Hp film is located at the reflection points of the obliquely propagating bulk acoustic wave radiated by LSAW. Typically, the transmission loss increases and the phase decreases with increasing humidity. However, for certain sample configurations, we observed the phase increase with increasing humidity. The sensor exhibits very fast response on the order of 0.25 s to abrupt humidity variations. Employment of LSAWs enabled us to implement a novel sensor configuration with separated SAW excitation/detection and sensing surfaces, which is especially promising for use with liquids and aggressive media.