3-(Substituted)vinylcephem nuclei have been prepared by the reaction of 3-formylcephem derivatives with stabilized phosphoranes. Appropriate synthetic steps allowed preparation of a series of 3-ethoxycarbonylvinyl- and 3-carboxyvinylcephem derivatives bearing a variety of 7-acylamino functions. The phenoxyacetyl and thiopheneacetyl derivatives of the 3-cyanovinylcephem nucleus were also prepared. Although general gram-positive activity was comparable to cephalothin in many cases, against penicillin G resistant Staphylococcus aureus, the new cephalosporins were of low effectiveness. The 3-(substituted)vinyl cephalosporins had good activity against a number of gram-negative organisms. In some cases, this activity was excellent. The N-acetyl analogs had surprisingly good activity relative to N-acetyl-7-ACA. The phenylmalonoyl side-chain derivatives were shown to have an unusual antibacterial spectrum expansion (relative to previously known cephalosporins) to include activity against Serratia marcescens and Pseudomonas aeruginosa.