The spin-label method was used to study the structure and molecular motion of polystyrene chains adsorbed on three different porous silica surfaces, in order to examine the effect of surface geometry on the conformational structure. Three kinds of spin-labelled polystyrene (PS) samples with narrow molecular weight distributions, having weight-average molecular weight (M-w) = 2.98 x 10(3), 96.4 x 10(3), and 700 x 10(3) were adsorbed on the surface of porous silica gels in cyclohexane (C6H12) at 35 degrees C. Electron spin resonance (e.s.r.) spectra were observed at various temperatures after the samples were completely dried. It was found that the observed e.s.r. spectra were composed of two spectra arising from spin labels attached to ’train’ and ’loop’ (or ’tail’) segments on the porous silica surface. The remarkable change of the e.s.r. spectra with temperature was analysed by computer simulation and the transition phenomena from ’train’ segments to ’loop’ or ’tail’ segments were clarified. ’Loop’ (or ’tail’) segments of adsorbed PS, having M-w = 700 x 10(3) were also converted to ’train’ segments with increasing adsorption time when the chains penetrate into the inner part of the porous silica of small pore diameter. The conformational changes are strongly affected by the surface geometry, the size ratio of the average pore diameter to twice the radius gyration of a PS chain, and relaxation of the PS chain.