The molecular structure of 1,1-dichlorocyclopentane (DClCP) has been investigated by means of gas-phase electron diffraction and ab initio calculations. Although the electron diffraction data could be fairly well reproduced by a C, (envelope) model we found it more pertinent to apply a pseudorotation model to account for the dynamic and large amplitude motion in DClCP. On the basis of this model we analyzed the dependency of the ring geometric parameters and vibrational mean amplitudes on the phase angle phi. For a better elucidation of this distinct dependency we developed particular equations which describe the dependency of the distribution of the delocalized net charges throughout the ring on the phase angle phi. The joint electron diffraction and ab initio study has led to the following r(a) structural parameters of DClCP (C-s conformer): r(C-Cl)(ax) = 1.787(3)Angstrom, r(C-Cl)(eq) = 1.769(3) Angstrom, average r(C-C)(ring) = 1.535(1) Angstrom, r(C-H)(av) = 1.114(5) Angstrom, angle(C5 - C1 -C2) 103.0(9)degrees, angle(Cl - C - Cl) = 108.6(3)degrees, and angle(H-C-H) = 104.6(26)degrees. The puckering amplitude for the five-membered ring was determined to be q = 0.400(11) Angstrom. The quantum mechanical calculations were carried out by utilizing the Hartree-Fock, density functional B3PW91, and perturbation MP2 methods and applying the following basis sets: cc-pVDZ, 6-31G(d,p), 6-311G(df,pd), 6-311+G(d,p), 6-311++G(d,p), 6-311+ G(2df,2pd), and 6-311++G(2df,2pd). For the purpose of comparison and systematic study, we optimized the geometries and calculated the charge distributions using the natural population analysis (NPA) and Mulliken population analysis (MPA) of 1,1-difluorocyclopentane, 1,1-dibromocyclopentane, and their corresponding monohalogenated derivatives.