The influence of the microstructures of different κ-carrageenan gels on the self-diffusion behavior of poly(ethylene glycol) (PEG) has been determined by nuclear magnetic resonance (NMR) diffusometry and transmission electron microscopy (TEM). It was found that the diffusion behavior was determined mainly by the void size, which in turn was defined by the state of aggregation of the κ-carrageenan. The κ-carrageenan concentration was held constant at 1 w/w%, and the aggregation was controlled by the amount of potassium and/or sodium chloride and, for samples containing potassium, also by the cooling rate. Gels containing potassium formed microstructures where κ-carrageenan strands are rather evenly distributed over the image size, while sodium gels formed dense biopolymer clusters interspersed with large openings. In a gel with small void sizes, relatively slow diffusion was found for all PEG sizes investigated. Extended studies of the self-diffusion behavior of the 634 g mol-1 PEG showed that there is a strong time dependence in the measured PEG diffusion. An asymptotic lower time limit of the diffusion coefficient was found in all gels when the diffusion observation time was increased. According to the ratio, D/D0, where D0 is the diffusion coefficient in D2O and D is the diffusion coefficient in the gels, the gels could be divided into three classes: small, medium, and large voids. For quenched κ-carrageenan solutions with salt concentrations of 20 mM K+, 100 mM K+, or 20 mM K+/200 mM Na + as well as slowly cooled solutions with only 20 mM K+, D/D0 ratios between 0.18 and 0.29 were obtained. By quenching a κ-carrageenan solution with 100 mM K+, the D/D0 was 0.5, while D/D0 ratios between 0.9 and 1 were obtained in a quenched solution with 250 mM Na+ and slowly cooled samples with 20 mM K +/200 mM Na+ or 250 mM Na+.