The exchange of trace gases across the air-sea boundary layer is characterized by the gas transfer velocity k = 1/β u* Sc-n,with the friction velocity u* (a measure for momentum input into the water), the tracers Schmidt number Sc, and the Schmidt number exponent n which changes gradually from n=2/3 for a smooth water surface, to n=1/2 for a rough and wavy surface. β denotes the momentum transfer resistance, a dimensionless scaling parameter. In recent years, many new insights into the transition of the Schmidt number exponent n from smooth to rough conditions were acquired in detailed lab studies, while there is a lack of studies focusing on β. Using experimental data from various wind-wave tank laboratories, we found that β, just like the Schmidt number exponent n, is not a constant but depends on the state of the water surface as well as the degree of contamination of the water surface with surface active material.