Cyclohexane and seven n-alkylcyclohexanes (alkyl side-chain CmH2m+1, m = 1, 2, 3, 4, 6, 8, 10) were pyrolyzed in or near the supercritical phase in a batch reactor at 450 degrees C under relatively high (greater than or equal to 2 MPa) and continuously increasing pressure for 6-480 min. The thermal stability of alkylcyclohexanes decreases with increasing side-chain length. The major reaction pathways of alkylcyclohexanes are strongly dependent on the side-chain length. For cyclohexane and methylcyclohexane, the dominant reaction is isomerization to form alkylcyclopentanes via ring contraction. The tendency to isomerization decreases with increasing side-chain length. For alkylcyclohexanes with m greater than or equal to 3, the major reaction at early decomposition stages is beta-scission, leading to C-C bond cleavage in the side-chain at or near the ring followed by H-abstraction. The decomposition resulted in three pairs of most abundant products: cyclohexane plus 1-CmH2m, methylenecyclohexane plus n-C-(m-1)H-2(m-1+2), and cyclohexene plus n-CmH2m+2. Under the conditions used, alkylcyclohexanes do not undergo ring-opening cracking to any significant extent. An empirical equation was developed to correlate the rate constant with the molecular structure of alkylcyclohexanes using a group contribution method.