Metal-organic chemical vapor deposition (MOCVD) is a suitable technique for the preparation of III–V epitaxial layers which are used in the fabrication of microelectronic and optoelectronic devices. The usual Ga and As sources for GaAs are Ga(CH3)3 and AsH3, respectively. However, the use of these precursors has some disadvantages including: The toxicity and storage of arsine, stoichiometry control, carbon incorporation and unwanted side reactions. Several groups of researchers have investigated alternative sources for both the group–III and group-V elements. A review of these new organometallic precursors is presented in this paper. However, because group-III and group-V elements form Lewis-acid/base adducts in the MOCVD reactor, we have especially investigated the use of this class of compounds as single starting molecules. Several adducts have been successfully used for the epitaxial growth of GaAs. Moreover, to avoid any stoichiometry loss due to dissociation of the adduct, the properties of organometallic molecules which feature a covalent bond between the group-III and group-V elements have also been investigated. These covalent compounds are probably formed in the MOCVD reactor when alkyl group-V compounds containing acidic hydrogen R3−nMHn (M = As, P; n = 1,2) are used. Such new precursors are also briefly reviewed.