The mechanism of acid-catalyzed hydrocarbon reactions such as skeletal isomerization, hydride transfer, alkylation, dehydro,enation and cracking in superacid media and over solid zeolites is analyzed and compared. In particular, the stability, rearrangements and nature (reaction intermediates or transition states) of trivalent carbenium ions and non-classical pentacoordinated carbonium ions in homogeneous and heterogeneous phase are discussed. It is concluded that both carbenium and carbonium ions may exist as true reaction intermediates in zeolite-catalyzed processes only when the positive charge is not easily accessible to framework oxygen atoms. (c) 2007 Elsevier B.V. All rights reserved.