o-Cresol-d3 is widely used as an internal standard in analytical chemistry, particularly in mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Its deuterated form allows for precise quantification and identification of cresol isomers in complex mixtures, such as environmental samples, biological fluids, and industrial products. In environmental studies, it aids in tracking phenolic pollutants and their degradation pathways. Additionally, it serves as a labeled compound in metabolic research to study the biotransformation of cresols in biological systems, providing insights into their toxicological and pharmacokinetic properties. Its stability and distinct isotopic signature make it a valuable tool in high-precision analytical applications.