Internet access using relatively fast Wireless Local Area Networks (WLANs) is often perceived as a local complement to slower, but widely available access using cellular networks, e.g. GSM/GPRS or UMTS. To benefit from the advantages offered by both radio access networks, a mobile user should be able to seamlessly change the access network while moving, experiencing temporary network congestions or whenever switching to the other network seems profitable. The goal of this paper is to analyse the profitability of performing vertical handovers between IEEE 802.11b and UMTS in case of using Mobile IPv4. Such research has not yet been done in detail before. The profitability of performing vertical handovers has been analysed depending on the available UMTS channel bandwidth, channel quality, number of WLAN users and generated traffic type. Several detailed simulations have been carried out using NS2 and results are presented. Based on these results, a general criterion for profitable switching between UMTS and IEEE 802.11b is proposed, analysed and verified by simulations.