It is shown that spectral anal. of non-intrusive time dependent pressure measurements in bubbling and circulating gas-solid fluidized beds permits to obtain the first ests. of bubble, gas slug, and solids cluster length scales from pressure fluctuation data. These length scales are calcd. from the incoherent cross power spectra of pressure signals measured in the bubbling or circulating bed and in the plenum. Remarkable quant. agreement with bubble diam. data is found, and equally remarkable agreement is obtained with independent ests. 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.