Abstract The hot molecule (S0**) is in a highly vibrationally excited state formed by rapid internal conversion from initially prepared electronic excited states. The importance and predominance of the S0** mechanism in VUV gas phase photochemistry has been demonstrated. In this account, the fundamental physicochemical properties of S0** are discussed in order to understand the characteristic features of S0** reactions, and VUV laser chemistry is reviewed from the following two points of view: 1) The generalization and classification of VUV chemistry for a variety of molecules, including the hot molecule mechanism and other competitive mechanisms; 2) A new strategy of multiphoton chemistry that employs hot molecule as an intermediate. Internal conversion is a dominant deactivation process for many molecules in the VUV region. Aromatic hydrocarbons and olefins are the representative examples of S0** reactions. However, fluorescence and intersystem crossing are major deactivation processes in some cases such as large condensed aromatic hydrocarbons. The reactions of chlorinated compounds and phenols are explained by other reaction mechanisms such as direct dissociation and predissociation. Carbonyl compounds and some amines are classified into the intermediate cases. Due to the large molar extinction coefficient and relatively long lifetime of S0**, the multiphoton absorption process can be induced by a single nanosecond laser pulse. As a result of fast internal conversion and intramolecular energy redistribution, the second photon and further photons will not be absorbed by the molecule in the electronic excited state but by that in the S0**, and the photon energy of the multiphoton is accumulated as vibrational energy. Therefore, ionization is minor process in the case of the multiphoton reaction of S0**. The neutral radical formation is predominant in the dissociation reaction, and the rate constant increases because the internal energy is multiplied by the multiphoton absorption process. The applications of these findings lie in the following: 1) Large molecules of which the single-photon hot molecule reaction rate is small; 2) Molecules which have been deemed to be photoinert.