Poly(methyl methacrylate) (PMMA) gels have been prepared with radical polymerization by cross-linking methyl methacrylate monomer using ethylene glycol dimethacrylate as cross-linking monomer in toluene containing polystyrenes (PSs) with M-w from 4000 to 400,000. The diffusion coefficients of the PSs in the PMMA gels swollen in deuterated chloroform have been measured by pulsed field-gradient (PFG) H-1 NMR method with the diffusing time Delta varied. From the experimental results, it is found that the network structure of PMMA gels prepared in the presence of PSs with M-w = 4000 and 400,000 are relatively homogeneous and inhomogeneous, respectively, within the Delta range from 40 to 500 ins. (c) 2005 Elsevier Ltd. All rights reserved.