This paper investigates the effect of income diversification on the performance of Islamic banks in Malaysia, Saudi Arabia, Kuwait, United Arab Emirates, Bahrain and Qatar where they operate alongside conventional banks in a dual banking system. Accounting data was drawn from 68 conventional and 42 Islamic banks from 1997 to 2009. The main focus was to see whether a greater reliance on non-financing income impacts on earnings quality and if so, how this may vary between Islamic and conventional banks. Commission and fee income, trading income and other non-financing income constitute non-financing income. For conventional banks, this is known as non-interest income, but in Islamic banking the payment and receipt of interest is prohibited so this 'other income' is referred to as non-financing income (that is, income unrelated to deposit-taking and loan granting). Islamic banks operate as universal banks and offer retail and wholesale financing plus investment banking services. Using various empirical approaches, we find that non-financing income positively influences banks' risk-adjusted performance on a net overall impact basis. Greater income diversification on its own, increases income volatility and this negatively impacts bank's risk-adjusted performance. Islamic banks are found to be more focused on deposit/loan financing and less diversified in terms of non-financing income activities compared to conventional banks. We find that Islamic banks appear to be less susceptible to earnings volatility given their lower diversified income source. Islamic banks have lower profitability (on average) on a risk-adjusted basis when compared to their conventional counterparts.