The environment humidity effects on performance of halide perovskites (HaPs), especially MAPbI 3 , are known. Nevertheless, it is hard to find direct experimental evidence of H 2 O in the bulk materials at the levels lower than that of Monohydrate (MAPbI 3 .H 2 O). Here, for the first time, direct experimental evidence of water being released from bulk (µm-s deep) of MAPbI 3 single crystal is reported. The thermogravimetric analysis coupled with mass spectrometry (TGA-MS) of evolved gases is used to detect the MS signal of H 2 O from the penetrable depth and correlate it with the TGA mass loss due to H 2 O leaving the material. These measurements yield an estimate of the average H 2 O content of 1 H 2 O molecule per three MAPbI 3 formula units (MAPbI 3 .0.33H 2 O). Under the relatively low temperature conditions no other evolved gases that can correspond to MAPbI 3 decomposition products, are observed in the MS. In addition to being direct evidence that there is H 2 O inside MAPbI 3 , the data show that H 2 O diffuses into it. With this article, a solid basis is proved for further studies on the mechanisms through which water modifies the properties of MAPbI 3 and all the other halide perovskites.