Farmers often mix fertilizers, pesticides and other agricultural chemicals together in a spray tank to allow applications of multiple products in a single spray. Because polyborate-based B fertilizers may increase solution pH, adding B to tank-mixed sprays may impair the stability and efficacy of alkalinity-sensitive pesticides and growth regulators if an acidifier is not included. We conducted a laboratory experiment to determine the influence of 10 commercial B fertilizer sources in factorial combination with B concentrations ranging from 0 to 4 lb/100 gal (4.8 g·L-1) on solution pH values of distilled water and two natural waters. Two boric acid-based compounds produced acidic reactions relative to background water pH at all tested B concentrations. Their pH responses were influenced by initial water composition. Seven B products produced moderately to strongly alkaline reactions at all but the highest B concentration, regardless of the form of B (polyborate vs. boric acid) initially present in the formulated products. One polyborate product formulated with an acidifier showed intermediate pH behavior. The dependence of solution pH on B rate of the polyborate-containing products was identical in all three waters. The maximum pH values generated by all products occurred in the B concentration range <0.1 to 0.25 lb/100 gal (0.12 to 0.3 g·L-1). Solution pH values declined with increasing B concentration above this range. The pH responses qualitatively conformed to known aqueous chemical behavior of B and the product additives. The complexity of the interaction between initial water chemistry, B concentration, and B fertilizer product reinforces the need to measure the pH of B-amended spray water before adding pH-sensitive compounds.