As part of a broader research program dealing with initial atmospheric corrosion of metals induced by organic acids, infrared reflection absorption spectroscopy (IRAS) has been used to study the initial interaction of acetic acid with zinc in 90% relative humidity and 120 ppb acid concentration under in situ condition. Efforts were made to quantify the identified reaction product species, mainly zinc oxide and zinc acetate. The experimentally obtained results were quantified through calculations using a linear relationship between IRAS peak intensity and layer thickness, and assuming optical constants of zinc acetate dihydrate (Zn(CH 3COO) 2•2H 2O). In all, the in situ studies performed by IRAS provide information on the identity and quantity of reaction products formed. © 2009 by NACE International.