Selective ring opening is an important hydrotreating process for gas oil upgrading. In this work, we have used an Ir/NaY bifunctional catalyst-highly efficient in sulfur-free conditions-as a reference system to assess the effect of H2S impurity concentration (0-1 %) on the kinetics and mechanisms of naphthene conversion under high hydrogen pressure (5 MPa). Three model naphthenic molecules (decalin, perhydroindan and butylcyclohexane) were compared to evaluate the influence of the ring size (C5 vs C6) and number (1 vs 2). The numerous reaction products were identified, quantified and classified by using two-dimensional gas chromatography (GC x GC). In the absence of sulfur, it is confirmed that C5 rings are opened faster than C6 rings, and that single-ring naphthenes are converted faster than double-ring naphthenes. The presence of H2S, even at concentrations as low as 30 ppm, drastically and irreversibly changes the dominant catalytic function (from metallic to acidic), mechanism (from dicarbene-mediated to carbocation rearrangement) and family of products (from ring-opening products to skeletal-isomerization products). Together with experiments at variable reactant conversion, these results allow us to propose mechanistic reaction schemes for the three naphthenes under similar conditions, both for sulfur-free and sulfur-rich atmospheres.