The crystal structure, thermal vibrations and electron density of L-arginine phosphate monohydrate (formally C6H15N4O2
+.H2PO4
−.H2O) have been analysed using 130 K single-crystal X-ray diffraction data to a resolution of (sin θ/λ)max = 1.20 Å−1. A multipolar pseudo-atom density model was fitted against the 6805 observed data with I > 3σ(I), [R(F) = 0.016,Rw
(F) = 0.014, S = 1.39] in order to map the static valence-electron density distribution. Positional and thermal vibration parameters for H atoms were taken from neutron diffraction results. A comparison between the electron density ρ(r), ∇2
ρ(r) and the electrostatic potential calculated from X−X and X−(X + N) refinements shows that reliable results may be obtained from X−X data only.