All commercial kinetic hydrate inhibitor (KHI) formulations are based on polymers with amide (or imide) functional groups. In our continuing work to explore non-amide-based KHI polymers, a series of poly(alkyl ethylene phosphonate)s (PPns) has been synthesized. These polymers were investigated for their performance as KHIs in high-pressure rocking cells using a structure-II-forming gas mixture and the slow constant cooling test method over 24 h. All of the PPns gave better KHI activity than tests with no additive. However, despite several of the polymers being designed to have low cloud points, a factor that is often useful for good KHI performance, none of the PPns gave lower onset temperatures than poly(N-vinylcaprolactam), a well-known commercial KHI with a low cloud point. A random PPn copolymer with eight ethyl and eight n-hexyl groups gave a biodegradation of about 31% using the marine OECD 306 test protocol.