Abstract: A thermosensitive recording medium comprising a thermosensitive recording layer containing an electron-donating color forming substance, an electron-accepting substance, and a color developability-improving agent is disclosed, said color developability-improving agent being a solid solution composed of two or more compounds containing at least one color developability-improving compound and having a single intrinsic melting point of not less than room temperature, preferably 60.degree. C. or higher. The recording medium exhibits well-balanced performance among color developing sensitivity, static color developability, image stability, and stability before recording.

Abstract: A thermosensitive recording sheet comprising a substrate sheet and coated on the substrate, a film comprising a color-forming lactone compound, an acidic substance and a sensitizer, said sensitizer being a substituted or unsubstituted phenacyl ether compound or a substituted or unsubstituted phenacyl sulfide compound.

Abstract: A novel thermosensitive recording sheet having a coated film containing a color-forming lactone compound, an acidic substance and a sensitizer, said sensitizer being an o-hydroxy aromatic acid ester represented by formula ##STR1## wherein R.sub.1 denotes an alkylene group, a cycloalkylene group, an oxyalkylene group, a thioalkylene group, a phenylene group, a xylylene group or a napthylene group, R.sub.2 denotes ##STR2##R.sub.3 denotes ##STR3## in which X denotes a hydroxy group or a hydrogen atom, provided a benzene ring and a naphthalene ring in R.sub.1, R.sub.2 and R.sub.3 may be substituted by a lower alkyl group, a nitro group or a halogen atom.

Abstract: A thermosensitive recording sheet having a coated film containing a color-forming lactone compound, an acidic substance and a sensitizer, said sensitizer being a sulfur-containing dibasic acid ester represented by formula (I) ##STR1## wherein n is an integer of 1 to 3, m is an integer of 1 or 2, R.sup.1 and R.sup.2 may be the same or different and each denotes an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group, an aryl group, --C.sub.l H.sub.2l --Ar or --C.sub.l H.sub.2l --CO--AR in which l is an integer of 1 to 8 and Ar denotes an aryl group, provided the alkyl group having 1 to 20 carbon atoms may be substituted by a halogen atom, and the cycloalkyl group, the aryl group and Ar of --C.sub.l H.sub.2l --Ar or --C.sub.l H.sub.