High free volume, film-forming copolymers were prepared in which a proportion of the spiro-units of PIM-1 were replaced with units derived from 9,10-dimethy1-9,10-dihydro-9,10-ethanoanthracene-2,3,6,7-tetrol (COI) A full investigation of free volume, utilizing N-2 sorption, positron annihilation lifetime spectroscopy (PALS). Xe sorption and Xe-129 NMR spectroscopy, was undertaken for copolymer PIM1-CO1-40 (spiro-units CO1 = 60 40) and a comparison is made with PIM-1 All techniques indicate that the copolymer, like PIM-1, possesses free volume holes or pores on the nanometre length scale (ie, microporosity as defined by IUPAC) For the batch of PIM-1 studied here, the sample as received showed anomalous N-2 sorption, Xe sorption and Xe-129 NMR behavior that could be interpreted in terms of reduced porosity in the size range 0 6-0 7 urn, as compared to the copolymer The anomalous behavior was eliminated on conditioning or relaxation of the polymer, e g, by Xe sorption at 100 degrees C and 3 bar PALS for both PIMI-CO1-40 and PIM-1 indicates a maximum in the average free volume hole size, and in the width of the distribution of hole sizes, on increasing temperature This maximum appears to be a feature of high lice volume polymers and may be related to the onset of localized oscillations of backbone moieties