The problems of structure and reactivity of the azides are of course closely related to their thermochemistry. Lattice energies, electron affinities and bond energies are especially important. Although numerous investigations have been made both into slow thermal decomposition and explosive decomposition of the azides interpretation has been hampered by lack of reliable thermochemical data. In the literature, only a collection of inconsistent and unreliable data for the heavy-metal azides is available; for the alkali-metal and alkaline-earth azides there are no data at all. This paper deals with the thermochemical relations of the azides and their application to reactivity. In part I the experimental determination of consistent enthalpy data for hydrazoic acid, the aqueous azide ion, the alkali metal, the alkaline-earth and heavy-metal azides is described. The values of the enthalpies of formation (∆
H
o
f
in kcal mole
-1
) are: N
-
3 Aq
(55∙51 H
2
O), 65∙53; HN
3G
, 71∙66; HN
3L
, 64∙37; LiN
3
, 2∙58; NaN
3
, 5∙08; KN
3
, 0∙33; RbN
3
, -0∙07; CsN
3
, -2∙37; NH
4
N
3
, 26∙79; CaN
6
, 11∙03; SrN
6
, 1∙72; BaN
6
, -5∙32; CuN
3
, 67∙23; CuN
6
, 140∙4; AgN
3
, 74∙17; Hg
2
N
6
, 141∙5; T1N
3
, 55∙78; PbN
6
, 115∙5. From these and other measurements consistent values for free energies and entropies of the azides are derived. These primary thermodynamic data will be employed in part II of this paper to derive important thermochemical quantities not susceptible to direct measurement such as bond dissociation energies, lattice energies and the electron affinity of the azide ion.