The properties of poly-N-vinylcaprolactam organic and aqueous solutions were studied. Toluene solutions present an increasing [η] from 10 to 40°C. Aqueous solutions present a very accentuated diminishing of [η] from 10 to 30°C. Above 30°C. The polymer precipitates from its aqueous solutions. The precipitation temperature does not depend on the concentration but on the molecular weight of the polymer. Some acid and basic agents have a marked influence on [η] and the precipitation temperature of the aqueous solutions.