The crystals of several derivatives ofN-salicylideneaniline which contain NHO hydrogen bonds in the molecule show thermochromism. To investigate the relation between the thermochromism and the proton transfer in the hydrogen bond, the proton dynamics of remarkably thermochromic crystal of BSP was precisely studied by solid state NMR method. The behavior of proton dynamics is good consistent with the temperature dependence of visible absorption spectra of this crystal. This result indicates that the thermochromism of this material is a prototype of “proton-electron cooperation”. The transfer rate of proton in the NHO hydrogen bond was strongly affected by some chemical modifications,e.g.the fast tunneling in one case.