The main aim of this paper is to provide insights into the performance degradation caused by interference between coexistent 802.11b and 802.11g WiFi systems operating in environments where channel overlap cannot be excluded. The fundamental differences between their interference characteristics are overviewed and analysed, and it is shown that channel overlap degrades their performance by differing amounts. It is also shown that for similar interference levels this degradation is slightly worse than for systems that are exclusively 802.11b but slightly better than for ones that are exclusively 802.11g. Overall, it is shown that for WiFi systems operating in environments where channel overlap exists, increasing the channel separation is generally of greater benefit for 802.11b than 802.11g.