Electrospun fibrous polyacrylic acid (PAA) membranes were studied as sensing material coated on quartz crystal microbalance (QCM) for ammonia detection at ppb level. The fibrous membranes with different morphology can be deposited on the electrode of QCM by electrospinning the PAA solutions with various solvent composition of H2O and ethanol. The results of sensing experiments indicated that the sensitivity of :he fibrous membranes coated QCM (FM-QCM) sensors was four times higher than that of the casting films coated QCM (CF-QCM) sensors. Meanwhile, the FM-QCM sensors exhibited high sensitivity towards low concentration of ammonia, as low as 130 ppb at the relative humidity of 40%. The pre-sorbed water in fibrous membranes was proved to be the key factor to affect the sensitivity of FM-QCM sensors for ammonia. The sensor performance has been found to depend on the morphology of the fibrous membranes, concentration of ammonia, coating load of the fibrous membranes on QCM, and relative humidity. Preliminary study of the stability investigation was also presented. © 2004 Elsevier B.V. All rights reserved.