The formation of secondary Ln(Ⅲ) solid phases (e.g., Nd2(CO3)3 and Sm2(CO3)3) was studied as a function of the humic acid concentration in 0.1 mol/L NaClO4 aqueous solution in the neutral pH range (5-6.5). The solid phases under investigation were prepared by alkaline precipitation under 100% CO2 atmosphere and characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), time-resolved laser fluorescence spectroscopy (TRLFS), diffuse reflectance ultraviolet-visible (DR-UV-Vis), Raman spectroscopy, and solubility measurements. The spectroscopic data obtained indicated that Nd2(CO3)3 and Sm2(CO3)3 were stable and remained the solubility limiting solid phases even in the presence of increased humic acid concentration (0.5 g/L) in solution.Upon base addition in the Ln(Ⅲ)-HA system, decomplexation of the previously formed Ln(Ⅲ)-humate complexes and precipitation of two distinct phases occurred, the inorganic (Ln2(CO3)3) and the organic phase (HA), which was adsorbed on the particle surface of the former.Nevertheless, humic acid affected the particle size of the solid phases. Increasing humic acid concentration resulted in decreasing crystallite size of the Nd2(CO3)3 and increasing crystallite size of the Sm2(CO3)3 solid phase, and affected inversely the solubility of the solid phases.However, this impact on the solid phase properties was expected to be of minor relevance regarding the chemical behavior and migration of trivalent lanthanides and actinides in the geosphere.