In this work a kinetic analysis based on the two-step reaction and Langmuir–Hinshelwood mechanisms is applied to the aqueous phase hydrodechlorination of 4-chlorophenol catalyzed by unsupported Rh nanoparticles. The theoretical predictions of the models based on the two-step and Langmuir–Hinshelwood methods showed good agreement with experimental activity data (turn over frequency) from reaction runs carried out at 303 K with nanoparticles of different sizes (1.9–4.9 nm). The theoretical maximum activity (29–30 h−1) was predicted for nanoparticle sizes of 2.5–2.7 nm, very close to experimental values (ca. 28 h−1, 2.8 nm). The applicability of the two-step reaction and Langmuir–Hinshelwood mechanism to unsupported metal nanoparticles is proved and mechanistic information is obtained. An important difference in the Gibbs replacement adsorption energy (100–130 kJ mol−1) of hydrogen by chlorophenol has been found between edges and terraces.