Mica multilayer thin films were assembled layer-by-layer (LBL) from mica that was modified with carboxylic groups (COOH) and cationic polyelectrolyte (poly(allylamine hydrochloride)) (PAH) on a quartz-crystal microbalance (QCM) for low-humidity sensing applications. The surface characteristics of the thin films were studied for various assembly conditions of pH and ionic strength of modified mica solutions. The thin films were characterized using QCM and by atomic force microscopy (AFM). The thickness of mica-COOH/PAH multilayer thin films increased with assembly pH of modified mica solution. When salt was added, the multilayer films became thicker, smoother and grew more regularly. The mica-COOH/PAH multilayer films that were deposited from the mica-COOH solution with salt at pH 3 was coated on the gold electrode of the QCM exhibited excellent sensitivity (0.524 −ΔHz/Δppmv at 36.1 ppmv) and linearity and a short response time (13 s at 60.4 ppmv).