In this paper we show that spectral analysis of non-intrusive time dependent pressure measurements in bubbling and circulating gas-solid fluidized beds permits to obtain the first estimates of bubble, gas slug, and solids cluster length scales from pressure fluctuation data. These length scales are calculated from the incoherent cross power spectra of pressure signals measured in the bubbling or circulating bed and in the plenum. Remarkable quantitative agreement with bubble diameter data is found, and equally remarkable agreement is obtained with independent estimates of gas slug lengths by others in circulating fluidized beds. These results demonstrate the possibility of greatly expanding the information that can be obtained non-intrusively from gas-solid fluidized beds. (C) 2002 Published by Elsevier Science Ltd.