Spin-labeled poly(acrylic acid)(SL-PAA) was synthesized and electron paramagnetic resonance (EPR) spectra of its dilute aqueous solutions were measured at different degrees of neutralization a in the temperature range 273-353 K. The temperature dependence of parameters characterizing the segmental rotational mobility of SL-PAA and the internal rotation of the spin label relative to the polymer chain were determined by simulating the line shapes of the experimental EPR spectra. The segmental rotational dynamics of SL-PAA was found to be nearly independent of its degree of neutralization. The absence of the methyl group is probably responsible for higher segmental rotational mobility of SL-PAA at all degrees of neutralization and in the entire temperature range studied, when comparing with spin-labeled poly(methacrylic acid) (SL-PMA) under the same conditions. SL-PAA forms strong complexes with both poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP) in aqueous solutions at alpha = 0.0 and 0.1. The complexes immediately precipitate from the solution under our experimental conditions. Complexation of SL-PAA neutralized to higher degrees (alpha = 0.2-1.0) with PEO or PVP in aqueous solutions was not observed.