This study focuses on the experimental investigation of the two-phase pressure drop in a thin mixed-wettability microchannel. Air-water flows in a thin microchannel of dimensions 3.23 mm wide by 0.304 mm high. The test conditions primarily produce rivulet flow. The two-phase pressure drop increases when the base contact angle changes from 76 degrees to 99 degrees, with the other walls remaining the same. Combining the result with existing literature demonstrates that consistent behavior in the change of the two-phase pressure when comparing different wettabilities arises with careful consideration of the experimental parameters to classify experiments of adiabatic two-phase flow in a single microchannel into three categories: homogeneous, hydrophobic mixed-wettability, and superhydrophobic mixed-wettability microchannels. The two-phase pressure measurements also allow for the assessment of homogeneous, separated, and relative permeability models. Limiting the analysis to the rivulet flow regime allows for the determination of a new relative permeability exponent of 1.747 in the two-fluid model, which produces a mean absolute percent error of 14.9%. However, the models do not fully collapse the data, indicating differing air-water interactions. The work discusses possible causes of this behavior from experimental limitations to instabilities of the rivulet flow. Published by Elsevier Ltd.