The microstructure and rheological properties of swellable nano-particles consisting of cross-link methacrylic acid/ethyl acrylate were examined using dynamic light scattering (DLS) and rheological techniques. A unified semi-empirical approach for predicting the viscosity of dilute and concentrated systems was proposed, where a variable specific volume, k was introduced to convert the mass concentration to effective volume fraction. The viscosity data for four different microgel systems showed excellent agreement with the modified Krieger-Dougherty model. The parameters, c(0) and m for the expression, k - k(min)/k(0) - k(min) = [1 + (c/c(0))(2)](-m) as a function of degree of neutralization (alpha) were determined. We observed that c(0) decreases with alpha, while m increases with alpha. The validity of the new approach was compared against the theoretical model for determining the effective volume fraction, phi = ((Q) over tilde /V(m)c(0))(rho(s)/rho(p))c and excellent agreement was observed. (C) 2004 Elsevier Ltd. All rights reserved.