The role of the C-OH group in citric acid (CA) in the molecular coordination with Fe3O4 nanoparticles (NPs) has been elusive for a long time. In this study, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectral deconvolution and thermogravimetric analysis (TGA) have been used to quantitatively clarify its significance in CA adsorption and its corresponding conformation. The experimental results show that the coordination and the corresponding conformation are exclusively determined by COOH not C-OH at pH 3, where its adsorption behavior conforms to the Brunauer-Emmett-Teller (BET) multilayer model with a maximal monolayer coordination number of 2.1/nm(2). However, C-OH is involved in the coordination at pH 10, and CA conforms to the Langmuir monolayer model with 1.4/nm(2) as its maximal monolayer coordination number, which is more stable than the COOH-only coordination. Especially, the conformational transformation is observed for the first time at pH 3, where the CA molecules adjust their conformation upon elution to maximize the utilization of the available binding sites on Fe3O4 NPs. This finding deepens the understanding on the fundamental mechanism for the interaction between the C-OH and COOH groups containing the organic ligand and metal oxide.