Polymers including chromophores, which can be switched by light, have been studied extensively during the last years due to a host of potential applications which arise from the marked changes in physical properties on switching. Even though there is clear evidence that the free volume has a significant influence on the isomerization kinetics, the question of free volume changes on switching was only addressed recently. Using a pulsed low-energy positron beam the ortho-positronium lifetime tau(3) was taken as a very sensitive free volume probe, and no change in free volume was detected on isomerization in an azobenzene-polymethylmethacrylate (PMMA) copolymer containing about 8 wt % of the azobenzene moiety. Here, we report for the first time on free volume changes in an azobenzene-PMMA blend with an azobenzene moiety concentration as high as 40 wt %. Using the same pulsed low-energy positron beam, small but significant changes of tau(3) were observed between the structurally relaxed dark and the UV-illuminated states suggesting a decrease in free volume of the order of 10%. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 404-408, 2011