An app. and a tracer response method were developed to det. the mass transport parameters in impregnating a porous body in supercrit. CO2 (e.g. silica alco-gel with active compds. that fail to get in at the gel-sol step of prepg. a SiO2/TiO2 composite) in relation to CO2 flow rate, impregnant amt., and sample size. The impregnant pulse was introduced into supercrit. CO2 and tracer concn. was followed at the outlet by an on-line UV/VIS detector. The response was interpreted in terms of a mathematic model developed. Diffusion coeffs. of supercrit. CO2 were estd. at 10(-9)-10(-5) m(2)/S.