The structure of Al-Ag alloys containing Guinier-Preston (GP) zones has been examined experimentally by grazing-incidence small-angle scattering (GI-SAXS) and numerically by Monte Carlo simulation to examine the microstructures near their sample surfaces. The GI-SAXS intensities were measured at Photon Factory, Tsukuba, Japan. Single-crystalline Al-1.5at%Ag samples in (110) sheets were aged after electrochemical polishing. The average size of the GP zones was smaller when the penetration depth of the incident X-ray decreased. This is in agreement with the conventional picture of precipitation-free zone (PFZ) formation. Although there are still experimental difficulties in conjunction with the surface roughness, the change in the average radius has been interpreted from the kinetic aspect of depleted mobility and composition in a kinetic Ising model.