In this paper we report about catalyst deactivation during reactions of 1-pentene over acidic ultrastable Y zeolite in a fixed-bed reactor. The effects of reaction temperature and time-on-stream (TOS) on product distribution, conversion and coke formation are discussed. At this work various coke groups - coke precursors, hard coke as well as total coke amount - were followed with TOS. Furthermore, the effects of coking on 1-pentene reactions were also studied. Cracking and hydride transfer were the predominant reactions in the first minute of TOS which deactivated rapidly allowing isomerisation to become the main reaction afterwards. A steep increase of the amount of coke formed was observed initially which was in good correlation with the initial rapid deactivation. The hydrogen freed during this initial time from the formation of high a carbon-to-hydrogen (C/H) ratio coke components contributed to the formation of hydride transfer products. (C) 2008 Elsevier B.V. All rights reserved.