Reduced-charge samples were prepared from Li+- and Ni2+-saturated SAz-1 montmorillonite by heating at 150 and 300°C for 24 h. X-ray diffraction analysis showed interstratification of non-swelling and swelling interlayers in LiS150, while fully expandable interlayers were found in NiS150. Heating at 300°C caused collapse of the interlayers in LiS300 in contrast to NiS300, for which some expandable layers were interstratified in the pseudo-pyrophyllite structure. The infrared (IR) spectra of heated Li-SAz showed new OH-stretching and overtone bands near 3670 cm−1 and 7170 cm−1 (1395 nm), respectively, indicating creation of local trioctahedral domains containing Li(I) in the previously vacant octahedra. No evidence of OH groups in trioctahedral coordination was found in the spectra of heated Ni-SAz. Nickel is supposed to be trapped in the hexagonal holes of the tetrahedral sheets. Reduction of the layer charge substantially affected the extent of the dissolution of SAz-1 montmorillonite in HCl. The MIR and NIR spectra of unheated Li- and Ni-SAz showed a substantial degradation of their structure after acid dissolution. New bands observed at 3744 cm−1 and 7314 cm−1 (1367 nm) were assigned to the vibrations of Si-OH groups formed upon acid treatment. These bands are a means of checking the extent of acid attack on smectites, even in cases when no differences are observed in the 1300–400 cm−1 spectral region (traditionally used to monitor this process). Both the IR spectra and solution analysis revealed that development of non-swelling interlayers in heated montmorillonites substantially reduced dissolution of these samples. The results obtained confirmed that acid attack of the smectite structure occurred at both interlayer surfaces and edges. If the accessibility of the layers for protons is low due to non-swelling interlayers, the dissolution was slower and took place mainly from the particle edges.