Guaiacol appears as a viscous colorless oil, although aged or impure samples are often yellowish.

[5] The pure substance is colorless, but samples become yellow upon exposure to air and light.

[9] Lignin, comprising a major fraction of biomass, is sometimes classified according to the guaiacyl component.

Pyrolysis of lignin from gymnosperms gives more guaiacol, resulting from removal of the propenyl group of coniferyl alcohol.

[13] Guaiacol is also a useful reagent for the quantification of peroxidases, as in the presence of hydrogen peroxide these enzymes will catalyse with it the formation of tetraguaiacol,[14] a coloured compound that can be quantified by its absorbance at 420–470 nm, following the equation: Guaiacol is a precursor to various flavorants, such as eugenol.

[17] Guaiacol is produced in the gut of desert locusts, Schistocerca gregaria, by the breakdown of plant material.

Dietary sources of methoxyphenols overwhelm the contribution from inhalational exposures to woodsmoke.