Gold nanoparticles (AuNPs) have been extensively used for detecting arsenite, As(III). Many methods rely on a DNA aptamer that claimed to bind specifically to inorganic arsenic. In these cases, the focus was on arsenic binding to the aptamer, while the potential interactions between As(III) and the AuNP surface were ignored. Herein, a set of spectroscopic and isothermal titration calorimetry (ITC) experiments were conducted to measure the adsorption of As(III) by AuNPs and its competition with DNA adsorption. With 10 mM As(III), 18% of adsorbed DNA was displaced from AuNPs, while preadsorption of only 20 mu M As(III) inhibited DNA adsorption by around 50%. The affinity of As(III) on AuNPs is comparable to Br- and guanosine. ITC and Raman spectroscopy both indicated that only As(III) can be adsorbed, while As(V) had no measurable interactions with the AuNPs. Based on this understanding, a random DNA sequence was used and a similar colorimetric response in the presence of As(III) was observed. This study confirmed the affinity between As(III) and the gold surface. The As(III)/gold interaction is strong enough to affect DNA adsorption, and care should be taken to interpret the observations based on the color change of AuNPs for the detection of As(III).