Swelling of poly-methylmethacrylate (PMMA) in weak solvents—water, ethanol, and methanol was investigated by interferometry using a simple microfluidic sensor. A change of channel width by ±10 nm, comparable with the surface roughness of the polymer surface, is reliably detected and its temporal evolution was monitored in situ. Dynamics of polymer swelling over time periods from a few seconds to a few days were obtained. An optimized inexpensive microfluidic sensor design using hot embossing with gold sputtering is presented. Such a sensor can be used for high-fidelity adsorption–desorption interferometric sensing and its cost can be optimized for a single use.