Antimicrobial polymers based on poly(ethylene-alt-maleic anhydride) (PEMA) were prepared. Amination of poly(ethylene-alt-maleic anhydride) using diamines of different chain lengths such as ethylenediamine (EDA) and hexamethylenediamine (HMDA) afforded terminal amino groups on the polymers. Antimicrobial polymers were obtained by immobilization of benzaldehyde and its derivatives, which include 4-hydroxybenzaldehyde and 2,4-dihydroxybenzaldehyde onto amine-terminated polymers. The antimicrobial activity of the prepared polymers were examined against different types of microorganisms including Gram-positive and Gram-negative bacteria as well as some fungi. The obtained results revealed that the antimicrobial activity increased with increasing the number of phenolic hydroxyl group and with increasing the spacer length.