Acronym – Noise-Immune Cavity Enhanced-Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) combines frequency modulation spectroscopy with a high finesse cavity. The use of frequency modulation spectroscopy shifts detection to high frequencies (e.g., 500 MHz) where technical noise is very low. The high finesse cavity provides long effective path lengths and hence extreme sensitivity. An advantage of NICE-OHMS is that it allows the study of slow reactions in the presence of fast ones.