A common issue in biomicrofluidic systems is that fluidic channels may become contaminated when ampiphilic molecules adsorb to the hydrophobic channel walls. Desorption rates are often measured using optical methods, many of which require a chromophore or a fluorophore label. This paper describes label-free desorption measurements using a drop frequency sensor (DFS), a microfluidic sensor reported recently by our group. The DFS is based on a surfactant retardation effect which measures the drag of surface-active agents on microdroplets generated in a tee junction. We measure desorption curves of Tween, a small molecule surfactant, from the walls of a polydimethylsiloxane channels. Typical desorption times increase with Tween concentration, ranging from 45 to 324 seconds at Tween concentrations between 10 and 1000 ppm. The limit of detection is 10 ppm.