Pool boiling experiments have been carried out on a smooth and an enhanced tube, employing sub-surface tunnels, with pure propane, pure iso-butane and two mixtures of propane and iso-butane as working fluids. It is found that, the enhanced tube suffers much more severe heat transfer degradation in boiling of mixtures than the smooth tube. This substantial degradation of boiling heat transfer is thought to be due to the malfunction of the sub-surface tunnels which is supported by some very special bubble behaviors observed in boiling of mixtures on the enhanced tube, e.g. the sharp turn of bubble curvature near the wall, the generation of small bubbles, the jets of small bubbles on the bottom surface, etc.. The measured bubble dynamics data also prove that the amount of vapor generated in boiling of mixtures is lower than that in boiling of pure components.