AbstractThe environment humidity effects on performance of halide perovskites (HaPs), especially MAPbI₃, are known. Nevertheless, it is hard to find direct experimental evidence of H₂O in the bulk materials at the levels lower than that of Monohydrate (MAPbI₃.H₂O). Here, for the first time, direct experimental evidence of water being released from bulk (µm‐s deep) of MAPbI₃ single crystal is reported. The thermogravimetric analysis coupled with mass spectrometry (TGA‐MS) of evolved gases is used to detect the MS signal of H₂O from the penetrable depth and correlate it with the TGA mass loss due to H₂O leaving the material. These measurements yield an estimate of the average H₂O content of 1 H₂O molecule per three MAPbI₃ formula units (MAPbI₃.0.33H₂O). Under the relatively low temperature conditions no other evolved gases that can correspond to MAPbI₃ decomposition products, are observed in the MS. In addition to being direct evidence that there is H₂O inside MAPbI₃, the data show that H₂O diffuses into it. With this article, a solid basis is proved for further studies on the mechanisms through which water modifies the properties of MAPbI₃ and all the other halide perovskites.