Random copolymers of ethene with 1-octene were investigated by means of pressure-volume-temperature (PVT) measurements. Solubility parameters were calculated from the thermal expansion coefficients and isothermal compressibilities. While expansion coefficients are virtually independent of 1-octene content, compressibilities decrease with increasing 1-octene incorporation. The solubility parameters varied between 17.3 and 15.2 MPa1/2 when 1-octene incorporation was varied between 2 and 100 wt%. In the framework of the solubility parameter concept copolymers containing less than 95 wt% 1-octene were shown to be immiscible with isotactic poly(propene).