Certain leuco dyestuffs are known to exhibit a brilliant color by reaction with a phenolic developer; 2-[(4-hydro-xyphenyl)sulfonyl]-phenol (BPS) is a well-known developer for black leuco dyes such as 6'-(diethylamino)-3'-methyl-2'-(phenylamino)-spiro[isobenzofuran-1(3H), 9'-[9H]-xanhen]-3-one (ODB) in thermosensitive papers. Although BPS is a small molecule (molecular weight: about 250), it is thermally stable as characterized by a high melting point of about 184°C. Furthermore, the stability of BPS is also extended to the 1:1 "leuco/developer" system. In this connection, structure analysis of BPS has been carried out in order to elucidate the stability of BPS itself as well as its leuco/developer system. Then, we found that one BPS molecule is hydrogen bonded to four different neighboring ones, forming an OH···O two-dimensional hydrogen-bond network. This ensures a high thermal stability of BPS. On the other hand, in the ODB/BPS system, only one of the four hydrogen bonds in BPS is used for the formation of the 1:1 ODB/BPS colorant while the remaining three hydrogen bonds keep the network. As for the tinctorial strength in spin-coated ODB/BPS, the color intensity is found to be limited to about 70% of the maximum available value. This is because there is a fraction of ODB molecules, whose lactone ring is still closed owing to the steric hindrance.