In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR). They can be considered as esters of an unobserved tautomer phosphorous acid, HPO, with the simplest example being trimethylphosphite, P(OCH).  Some phosphites can be considered esters of the dominant tautomer of phosphorous acid (HP(O)(OH)).  The simplest representative is dimethylphosphite with the formula HP(O)(OCH). Both classes of phosphites are usually colorless liquids.