This paper presents local saturated and subcooled boiling heat transfer coefficients for flow in narrow conduits. Flow of Freon Rll, HCFC123 and CO2 in circular pipes with diameters of 0.92 and 1.95 mm over a wide range of heat fluxes, mass fluxes, qualities and system pressures is examined. In all cases the saturated boiling heat transfer coefficient is found to be a function of the wall heat flux and not of the mass flux or quality, indicating the dominant heat transfer mechanism is nucleate boiling. The saturated data are fitted well by the nucleate pool boiling correlation of Cooper. The subcooled data suggest that heat transfer is enhanced above the additive sum of forced convection and nucleate boiling components.