Presented herein is a mathematical model of the liquid pressure drop occurring in triangular grooves. The model considers the interfacial shear stresses due to liquid-vapor frictional interaction, and a dimensionless vapor-liquid interface flow number, L(v), is introduced. Results are reported for channel angles ranging from 20 to 60 degrees, contact angles from 0 to 60 degrees, and dimensionless vapor-liquid interface flow numbers of 0, 0.25, 0.5, 0.75, and 1. These results indicate that the friction factor Reynolds number product is strongly dependent upon the channel angle, the contact angle, and the dimensionless vapor-liquid interface flow number. The predicted results agree well with the documented and available experimental data for the cases L(v) = 1 and L(v) = 0.