An infrared near-field scanning optical microscope (NSOM) is used in a controlled environment chamber to detect water vapor uptake in thin polymer films. A chemically amplified photoresist sample composed of alternating 2 μm lines of the original 1000 nm thick poly-(tert-butylmethacrylate) (PTBMA) and the photochemically modified 500 nm thick poly(methacrylic acid) (PMAA) is studied both in low and high water vapor environments. The degree of water vapor absorption by the sample is measured using the infrared transmission of 2.85 μm light on a small spatial scale (<500 nm). The accompanying topographic swelling of the samples is measured using a shear-force feedback loop. Distortion of the topographic structure and variation in transmission contrast indicate that the PMAA zones absorb more water than the PTBMA regions in the water vapor environment. The PMAA swells 280 nm more than PTBMA when exposed to a partial pressure of water vapor of 2.1 kPa (16 Torr), whereas the infrared optical contrast is increased to a 6 ± 1% difference in the PMAA regions compared to the PTBMA.