Small-angle X-ray scattering (SAXS) was used to obtain solution parameters of a weak polyelectrolyte in water in the absence of any additives, such as neutralizing agents or salt. Poly(acrylic acid) (PAA) was used as a weak polyelectrolyte from which SAXS data were obtained in the dilute region of 1-10 mg cm(-3). An intrinsic viscosity of 15.7 dL g(-1) was obtained from a plot of reciprocal reduced viscosities versus the concentration. The application of the SAXS data, that is, the contour length (L = 1.97 X 10(4) Angstrom), the persistence length (a* = 58.5 Angstrom), and the molecular weight (M = 5.9 X 10(5) Da), to the Yamakawa-Fujii equation suggested that PAA in water at 25 degreesC could be described as a wormlike chain having a cylindrical body of d = 6 Angstrom. An end-to-end distance (r = 1.6 X 10(3) Angstrom) was calculated from r = 2a*L - 2(a*)(2). The nonisotropic expansion factor (alpha = 2.9) was calculated for PAA expanding from the random coil in dioxane at 30 degreesC (Theta temperature) to the wormlike chain in water at 25 degreesC. (C) 2003 Wiley Periodicals, Inc.