The removal of polynuclear aromatics from diesel fuel in response to increasingly stringent environmental legislation has stimulated intense research. In addition to the hydrogenation reaction that saturates aromatics, an interesting possibility is the opening of naphthenic rings derived from the saturation of aromatics to produce molecules of higher cetane numbers. In this contribution, we have investigated a series of HY and Pt/HY catalysts of varying acidity densities. Under the conditions of this study, mainly ring-contraction and one-ring opening reactions take place over these catalysts. Although the products from these reactions do not have cetane numbers significantly higher than those of the saturated aromatics, they can be important intermediates to high cetane number compounds, such as normal paraffins and some iso-paraffins. The results of this investigation show that HY zeolites can be effective catalysts for the ring contraction and one-ring opening of decalin if their acidity density is adjusted to an intermediate optimum, that is, high enough to achieve conversion, but not too high that would result in fast deactivation. The low cis-to-trans ratios obtained in the products are due both to the cis-to-trans isomerization, but more importantly, to a much higher reactivity of cis-decalin than trans-decalin. Also, cis-decalin converts much more selectively to ring-opening products than trans-decalin, which mainly converts to cracking products. The production of ring-contraction and ring-opening products from tetralin is greatly enhanced in the presence of Pt due to hydrogenation of tetralin to decalin. In the presence of hydrogen Pt/HY catalysts as well as physical mixtures of HY and Pt are much more effective than HY catalysts. (C) 2004 Elsevier Inc. All rights reserved.