Small strain Young's moduli of natural rubber (NR)/organoclay nanocomposites were estimated using the Guth-Gold, Halpin-Tsai (HT), and Krieger-Dougherty (KD) models, and compared with experimental measurements of NR vulcanizates containing organo-montmorillonite (OM) or organo-sepiolite (OS). To account for the effect on modulus of the NR matrix of the vulcanization-active modifier in the organoclay, a matrix modulus correction (MMC) term was derived from the vulcanization parameters of the nanocomposites. The KD model gave a better empirical fit with the experimental data than the Guth-Gold model, with both giving good agreement with particle shape factors estimated from transmission electron microscope (TEM) images. The HT model gave the best fit with experiment for both types of nanocomposite, and use of the MMC term meant that the empirical shape factor was sufficiently close to that estimated from TEM images that the model could potentially be used to accurately predict the Young's moduli of NR/OM and NR/OS nanocomposites. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1621-1627, 2011