Au nanoparticles with diameters ranging between 15 and 170 nm have been synthesised in aqueous solution using a seed-mediated growth method, employing hydroxylamine hydrochloride as a reducing agent. Thiolated polyethylene glycol (mPEG-SH) polymers, with molecular weights ranging from 2100 to 51 000 g mol⁻¹, were used as efficient particle stabilising ligands. Dynamic light scattering and zeta potential measurements confirmed that the overall mean diameter and zeta potential of the capped nanoparticles increased in a non-linear way with increasing molecular weight of the mPEG-SH ligand. Electron microscopy and thermal gravimetric analysis of the polymer-capped nanoparticles, with a mean gold core diameter of 15 nm, revealed that the grafting density of the mPEG-SH ligands decreased from 3.93 to 0.31 PEG nm⁻² as the molecular weight of the ligands increased from 2100 to 51 400 g mol⁻¹ respectively, due to increased steric hindrance and polymer conformational entropy with increase in the PEG chain length. Additionally, the number of bound mPEG-SH ligands, with a molecular weight of 10 800 g mol⁻¹, was found to increase in a non-linear way from 278 (σ = 42) to approximately 12 960 PEG (σ = 1227) when the mean Au core diameter increased from 15 to 115 nm respectively. However, the grafting density of mPEG_10 000-SH ligands was higher on 15 nm Au nanoparticles and decreased slightly from 1.57 to 0.8 PEG nm⁻² when the diameter increased; this effect can be attributed to the fact that smaller particles offer higher surface curvature, therefore allowing increased polymer loading per nm². Au nanoparticles were also shown to interact with CT-26 cells without causing noticeable toxicity.