Grazing incidence X-ray reflection has been used to examine the structure of spread monolayers of poly(N-alkyl-4-vinylpyridinium bromide)s, denoted by PC(n) where n is the length of the alkyl side chain, on water. Members of the series with even n from 8 to 22 were investigated, and the results for PC18 are compared with those of neutron reflection on the same sample. The comparison of neutron and X-ray results on PC18 makes it possible to identify those contributions to the X-ray reflectivity coming from the polymer backbone and alkyl chains for this particular polymer. On the basis of this comparison the thickness of the polymer layer has been estimated for each member of the series. The ratio of the observed thickness of the monolayer to its maximum possible value (delta/l) varies from 1.3 for n = 8 through a minimum of 0.7 at n = 16 to 0.8 at n = 22. Even when allowance is made for the contribution of capillary waves to the thickness of the layer, delta/l remains above unity for the shortest chain compounds. This is interpreted as arising from disorder in the layer. As the side chain length increases, it imposes order on the chain orientations. For small n an increase of n therefore decreases the thickness of the layer, but once the side chains are reasonably ordered the increase in chain length causes the thickness to increase once more. The X-ray reflectivity shows, independently of the neutron result, that part of the polymer layer protrudes out of the water. For small n this projection is about one-third of the thickness of the total monolayer thickness, but this proportion increases with n until eventually it has reached about the total thickness of the layer. Comparison with similar dimensions in surfactant layers and the PC(n) Langmuir-Blodgett films indicates similar behavior at small n. The comparison between LB films and the precursor monolayer of the PC(n) shows that disorder in the latter carries over into the LB film.