This paper (1) on the nature and properties of fumigants discusses (a) definitions and range of application, (b) relationships to atmosphereic pollutants and vapor phase migration of solid pesticides, (c) criteria of selection and evaluation of fumigants, with a note on toxic hazard, (d) factors that influence distribution and persistence of fumigants, and (e) sorption phenomena (physical sorption, chemisorption, fumigant residues, chromatographic properties of wheat and other cereals, sorption by other materials such as soil, steel, rubber, leather, fruit, etc.) Paper II to follow deals with sampling and microdetermination for fumigants in air and as residues.