The morphology and the chemistry of dental enamel and dentin can be modified by irradiation with a CO2 laser to increase the acid resistance of the intrinsic mineral. The changes induced in hard dental tissues after laser irradiation are predominantly determined by the photothermally induced temperature rise in the tissue. Therefore the temperature rise in the irradiated enamel and dentin must be determined under controlled laser conditions. Surface and subsurface temperatures were monitored after multiple-pulse CO2-laser irradiation at (lambda) equals 9.3, 9.6, 10.3, and 10.6 micrometers with 1- and 20-J/cm2 pulses of 50- to 500-microsecond(s) duration using radiometry and microthermocouples. Surface temperatures were significantly higher after 9.3- and 9.6-micrometers irradiation than for the more commonly utilized 10.6-micrometers CO2-laser wavelength. Permanent changes in the temperature response of enamel and dentin were observed at fluences greater than 2 J/cm2 and 100-microsecond(s) duration for dentin and 5 J/cm2 for enamel. CO2-laser irradiation changes the thermal and the optical properties of these tissues, substantially changing the energy deposition for subsequent laser pulses. These changes affect both the amount of energy absorbed and the depth of absorption. The more efficient absorption at (lambda) equals 9.3 and 9.6 micrometers may be advantageous for both caries-preventive treatments and ablation of exposed hard dental tissues while minimizing heat deposition in the tooth.