The coordination structure of the cobalt(II) ion and its chloro complexes of low concentration (∼20 mmol dm −3) in hexamethylphosphoric triamide (HMPA) has been explored by laboratory EXAFS (extended X-ray absorption fine structure) modified for fluorescence detection. The cobalt(II) ion is four-coordinated in HMPA, unlike other oxygen-donor solvents such as water and N,N-dimethylformamide (DMF). The CoO (HMPA) bond lengths within complexes are revealed to be 194(1), 195(1), 194(1) and 202(2) pm for [Co(hmpa) 4] 2+, [CoCl(hmpa) 3] +, [CoCl 2(hmpa) 2] and [CoCl 3(hmpa)] −, respectively. The CoO bond length is significantly shorter than that within six-coordinate [Co(H 2O) 6] 2+ (208 pm) or [Co(dmf) 6] 2+ (206 pm), and remains practically unchanged upon formation of mono- and dichloro complexes. On the other hand, the CoCl bond length is 224(1), 227(1) and 229(1) pm within [CoCl(hmpa) 3] +, [CoCl 2(hmpa) 2] and [CoCl 3(hmpa)] −, respectively, all of which are appreciably shorter than that within the six-coordinate [CoCl(H 2O) 5] + (235 pm). The origin of the different thermodynamic behavior of chloro complexation of cobalt(II) in HMPA from that in DMF will be discussed on the basis of these structural data.