This paper presents original experimental data regarding the rheological characterisation of nanofluids based on poly(ethylene glycol) s with molecular mass of either 200 or 300 g·mol⁻¹ (i.e. PEG-200 or PEG-300) and containing 0.025–0.70 wt% of multi-walled carbon nanotubes (MWCNT). The shear rate dependence of dynamic viscosity (η) was first studied in the temperature range from 278.15 to 313.15 K. Such non-linear viscoelastic experiments revealed that, unlike the neat poly(ethylene glycol)s used as based fluids, MWCNT/PEG-200 and MWCNT/PEG-300 nanofluids exhibited a non-Newtonian pseudoplastic behaviour in the low shear rate region (even in the case of the dispersions prepared at lowest nanoparticle concentrations). Dynamic viscosities measured at ~800 s⁻¹ were compared with the values provided by means of different theoretical or semi-empirical approaches and a good description of our experimental results was obtained (with absolute average deviations ~0.2–0.3%) by using the Maron–Pierce equation modified to take into account the presence of aggregates/agglomerates. Additionally, rheological oscillatory analyses of these nanofluids were also conducted at 288.15 K to identify the linear-viscoelastic region and deep in the analysis of the viscoelastic behaviour of designed dispersions.