The thermal Marangoni force on a bubble, as originally predicted and experimentally verified by Young et al., has been measured by at least three later investigators. In each of the later cases, the measurements were complicated by oscillations in the size of the bubbles, which produced vertical oscillatory motion of the bubble in the combined thermal and gravitational fields. The authors ascribed this motion to the variable solubility of air in the working liquid as the bubble moved in the temperature gradient. It is pointed out here that this variable solubility implies a second, solutal Marangoni effect, which reduces the thermal forces. Since the solubilities of all gases are a function of temperature, solutal forces will generally accompany thermal forces on gas bubbles.