A local density functional study of Si41O( Al31, H1)-substituted offretites is presented. Proton siting and dynamical properties are investigated within the First-Principles Molecular Dynamics method, using a periodically repeated unit cell. Results for monoaluminated offretites, e.g., with one Al per unit cell, show that the proton is located inside the channel of the zeolite, where it is accessible to incoming molecules for reaction. Calculated vibrational spectra of the framework, extracted from a dynamical simulation, reproduce experimental data well. The determined OH stretching frequencies show a rather weak dependence on the H1 position. A comparison of these frequencies with those of offretites containing three Al per unit cell does not indicate a significative change.