Heat transfer enhancement in mixed convection flow between parallel plates heated uniformly from below is investigated. Numerical computations demonstrate heat transfer enhancement prior to the onset of thermal instability in a horizontal channel due to fluid expansion and an induced pressure gradient, and in an inclined channel due to the component of gravity in the flow direction. Flow visualization and heat transfer measurements for airflow in a horizontal channel are performed over a wide range of conditions to delineate regions of forced and mixed convection. The onset of secondary flow is found to precede appreciable heat transfer enhancement. Except for large values Gr*/Re2, Nusselt numbers prior to the onset of enhancement may be predicted with forced convection correlations. Thereafter, Nusselt numbers rise rapidly to a maximum value and then assume a fully developed value which is slightly lower than the maximum. Correlations are suggested for the fully developed Nusselt numbers and for the locations of the onset of secondary flow, the onset of enhancement, and the maximum Nusselt number.