A microwave-induced acoustic (MIA) technique is described, which can be used to follow changes in the relative acidity of papers during the aging processes. Rapid aging is simulated by exposure of paper samples to atmospheres containing gaseous NOxand HCl. The MIA results correlate well with tensile strength measurements and are consistent with increasing acidity causing decreased fibre strength due to acid hydrolysis. The presence of some lignin content in papers appears to slow at least the initial detrimental effects caused by NOxexposure. The MIA technique is non-destructive and can be adapted to on-line measurements.