We present the results of a combined experimental and theoretical study of drop coalescence in the presence of an initial temperature difference ΔT₀ between a drop and a bath of the same liquid. We characterize experimentally the dependence of the residence time before coalescence on ΔT₀ for silicone oils with different viscosities. Delayed coalescence arises above a critical temperature difference ΔT_c that depends on the fluid viscosity: for ΔT₀ > ΔT_c, the delay time increases as ΔT₀^2/3 for all liquids examined. This observed dependence is rationalized theoretically through consideration of the thermocapillary flows generated within the drop, the bath and the intervening air layer.