Small-angle X-ray scattering (SAXS) measurements were carried out for two cellulose tris(phenylcarbamate) (CTPC) samples in tricresyl phosphate (TCP) at 25 A degrees C to determine the particle scattering function P(q) and the z-average mean-square radius of gyration . The obtained data were analyzed in terms of the wormlike chain model to estimate the Kuhn segment length lambda (-1) (the stiffness parameter, equivalent to twice the persistence length) and the helix pitch (or helix rise) per residue h. The resultant lambda (-1) and h were 11.5 +/- 0.5 and 0.51 nm, respectively. While the latter value (h) is consistent with the previously reported values both for cellulose and cellulose derivatives, appreciably higher chain flexibility was found for CTPC in TCP than that in tetrahydrofuran at 25 A degrees C (19-24 nm). The value is fairly close to that in anisol, cyclohexanol, and benzophenone, assuming an appropriate temperature coefficient. We may thus conclude that CTPC behaves as a semiflexible chain in TCP.