2-(Alkyloxycarbonyl)-2-alkylideneethyl esters of various aminothiazole-oxyimino cephalosporins have been synthesized and studied. They are useful alternatives to the currently existing orally active esters. Among the new esters synthesized, the 3'-azidomethyl cephem ester Ro 41-3399 (7k) presented an oral bioavailability superior to the corresponding pivaloyloxymethyl ester (9) in a rat model and was selected as a candidate for further evaluation.