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The transformation of n-heptane was studied at 250 °C, 1 atm, and PH/Pn-heptane = 9 on a series of PtHY catalysts containing from 0.1 to 1.5 wt % platinum and having Si/AI atomic ratios of 3, 9, or 35. The activities, stabilities, and selectivities of the catalysts are clearly governed by the number of their acid sites (nA) and hydrogenating sites (np,). nA is defined here as the number of acid sites on which the adsorption heat of NH3 is greater than 100 kj mol"1 and zip, as the number of accessible platinum atoms. For nA/n w <10 (i.e., with catalysts with about 1 wt % Pt and Si/AI atomic ratio of 9 or 35) the formation of monobranched, bibranched, and tribranched isomers and cracking products is a step by step process. Such catalysts present the best possible hydroisomerization selectivity, the cracking products appearing only at conversion rates of over 50% and the yield in isomers reaching 65% for a total conversion rate of 75-80%. Thus they can be considered as "ideal hydroisomerization catalysts".