Abstract. Results of research into four-membered phosphorus(V)-heterocycles, 2-halogen-1,2λ⁵-oxaphosphetanes are presented. The preparation of 2-halo-1,2λ⁵-oxaphosphetanes by reaction of P-haloylides with carbonyl compounds is described. The mechanism of asynchronous [2+2]-сycloaddition of ylides to aldehydes was proposed on the base of low-temperature NMR investigations. 2-Halogen-1,2λ⁵-oxaphosphetanes were isolated as individual compounds and their structures were confirmed by ¹Н, ¹³C, ¹⁹F and ³¹Р NMR spectra. These compounds are convenient reagents for preparing of various organic and organophosphorus compounds hardly available by other methods. Chemical and physical properties of the 2-halo-1,2λ⁵-oxaphosphetanes are reviewed. The 2-chloro-1,2λ⁵-oxaphosphetanes depending on substituents at the α-carbon atom rearrange with formation of 2-chloroalkylphosphonates or convert into trans-phosphorylated alkenes. Prospective synthetic applications of 2-halo-1,2λ⁵-oxaphosphetanes are analyzed. The 2-halo-1,2λ⁵-oxaphosphetanes may be easily converted to various alkenylphosphonates: allyl- or vinylphosphonates, phosphorus ketenes, thioketenes, ketenimines.