This paper describes a comparative study of residual heat deposition of Er:YAG and Er,Cr:YSGG lasers during ablation of hard dental tissues. Residual heat deposition was obtained from measured irradiated hard tissue temperature decay characteristics immediately following Erbium laser pulses. The measured residual heat was larger in enamel than in dentine for both Er:YAG and Er,Cr:YSGG sources. The amount of the unwanted residual heat that remains deposited in the tooth is for the H mode Er,Cr:YSGG by a factor of more than 2, and for the S mode Er,Cr:YSGG by a factor of more than 3 larger, compared to the deposited heat with the MSP mode Er:YAG laser. These findings, in addition to wavelength, pulse duration and pulse shape considerations made in previous studies,  at least partially explain the observed lower ablation efficacy of Er,Cr:YSGG lasers compared to Er:YAG lasers, and may lead to their reduced safety and comfort for patients.