Zero–valent copper engineered nanoparticles (Cu–ENPs) released through unintentional or intentional actions into the agricultural soils can alter the availability of inorganic phosphorus (IP) to plants. In this study, we used adsorption–desorption experiments to evaluate the effect of particle size of 1% Cu–ENPs (25 nm and 40–60 nm) on IP availability in Santa Barbara (SB) volcanic ash soil. X–Ray Diffraction results showed that Cu–ENPs were formed by a mixture of Cu metallic and Cu oxides (Cu2O or/and CuO) species, while specific surface area values showed that Cu–ENPs/25 nm could form larger aggregate particles compared to Cu–ENPs/40–60 nm. The kinetic IP adsorption of SB soil without and with 1% Cu–ENPs (25 nm and 40–60 nm) followed the mechanism described by the pseudo–second–order (k2 = 0.45–1.13 x 10−3 kg mmol−1 min−1; r2 ≥ 0.999, and RSS ≤ 0.091) and Elovich (α = 14621.10–3136.20 mmol kg−1 min−1; r2 ≥ 0.984, and RSS ≤ 69) models. Thus, the rate–limiting step for IP adsorption in the studied systems was chemisorption on a heterogeneous surface. Adsorption equilibrium isotherms without Cu–ENPs were fitted well to the Freundlich model, while with 1% Cu–ENPs (25 nm and 40–60 nm), isotherms were described best by the Freundlich and/or Langmuir model. The IP relative adsorption capacity (KF) was higher with 1% Cu–ENPs/40–60 nm (KF = 110.41) than for 1% Cu–ENPs/25 nm (KF = 74.40) and for SB soil (KF = 48.17). This study showed that plausible IP retention mechanisms in the presence of 1% Cu–ENPs in SB soil were: i) ligand exchange, ii) electrostatic attraction, and iii) co–precipitate formation. The desorption study demonstrated that 1% Cu–ENPs/40–60 nm increased the affinity of IP in SB soil with a greater effect than 1% Cu–ENPs/25 nm. Thus, both the studied size ranges of Cu–ENPs could favor an accumulation of IP in volcanic ash soils.