We have developed a light-scattering apparatus using a capillary tubing sample cell that is particularly suitable for investigating the structure of a single microcapsule. This approach is capable of determining the individual structures of polydisperse particles because the scattering function of each individual microcapsule can be measured independently. The apparatus was used to determine the outer diameter d and the membrane thickness delta of poly(L-lysine-alt-terephthalic acid) microcapsules as a function of pH. The values of both d and delta were found to be the lowest at pH 4 and increased as the pH value was shifted to either side of pH 4. The change in delta was roughly proportional to the change in d, suggesting an isotropic volumetric phase transition of the hydrogel membrane of the microcapsule. Diffusion coefficients of anionic and cationic electrolyte ions in the microcapsule membrane changed in parallel with delta.