The qualitative analysis of nanodiamonds by FTIR spectrometry as photoacoustic (PAS), diffuse-reflectance (DRIFT), and attenuated total reflection (ATR) modalities was evaluated for rapid and nondestructive analysis and comparison of nanodiamonds. The spectra reproducibility and signal-gathering depth was compared. The assignment of characteristic bands showed that only six groups of bands were present in spectra of all the modalities with appropriate sensitivity: 1760 (C=O stretch, isolated carboxyl groups); 1640–1632 (H–O–H bend, liquid water); 1400–1370 (non-carboxyl C–O–H in-plane bend and CH2 deformation); 1103 (non-carboxyl C–O stretch); 1060 (in-plane C–H bend, non-aromatic hydrocarbons and carbohydrates); and 940 cm–1 (out-of-plane carboxyl C–O–H bend). DRIFT provides the maximum number of bands and is capable of measuring hydrogen-bonded bands and CHX groups. ATR provides the good sensitivity for water and C–H/C–C bands in the range 2000–400 cm–1. PAS-FTIR reveals less bands than DRIFT but more intense bands than ATR-FTIR and shows the maximum sensitivity for absorption bands that do not appear in ATR-IR spectra and are expedient for supporting either DRIFT or PAS along with depth-profiling. Thus, all three modalities are required for full characterization of nanodiamonds surface functional groups.