Passive acoustic and vibrometric methods were investigated for the detection and monitoring of changes in bed fluidity in a large scale gas-solid fluidized bed after liquid injection. Water was injected into a 9 tonne air-fluidized bed of silica sand using an industrial nozzle and pre-mixer assembly. Acoustic signals were recorded using non-intrusive and externally mounted microphones. Vibration data was recorded using an accelerometer mounted to a rod inserted into the bed. The signals were analyzed offline using Fourier and wavelet analysis techniques. The average frequency of the acoustic and vibration measurements decreased as the bed solids dried while the standard deviation of the coefficients in the 10-20 kHz band characterized from wavelet analysis increased during drying. Samples from the bed were taken and evaluated for flowability by measuring the median avalanche time with a Revolution Powder analyzer. Linear and power law regressions of the wavelet and Fourier analyzed vibration measurements provided a reliable correlation of the flowability of the bed solids and thus could be successfully used as a passive and real-time monitoring method of bed fluidity. (C) 2010 Elsevier B.V. All rights reserved.