The effects on arsenic (As) removal at iron removal water treatment plants of two common chemical oxidants, potassium permanganate (KMnO4) and sodium hypochlorite (NaOCl), and ferric chloride (FeCl3) were characterized. On-site and laboratory experiments were performed using water from three Illinois water utilities whose potable water exceeds the As maximum contaminant level (MCL) of 10 mg L-1. Aeration, the most common iron removal process, was used as a point of reference. In all cases arsenic removal was improved (compared to aeration) by KMnO4 or NaOCl addition. Aeration had little effect on arsenic speciation; the proportions of As(III) and As(V) were essentially the same before and after aeration. Both KMnO4 and NaOCl oxidized As(III) to As(V). However, the KMnO4 dose depended only on the iron (Fe) concentration. On the other hand, NaOCl also reacts with ammonium (NH4 +) and the NaOCl dose depended on both Fe and NH4 + concentrations. Arsenic removal depended strongly on the Fe to As ratio with the best As removal at Fe to As values above 50. For the utility with the lowest As and highest Fe concentrations, oxidant addition alone was sufficient to satisfy the MCL. For the other two utilities both oxidant and FeCl3 had to be added to meet the MCL.