Electroabsorption (EA) studies at room temperature on organic thin films based on a dicyanovinyl-quaterthiophene 4T-V(CN)(2) are reported. An electric field modulation is applied to the samples for two different electrode geometries, i.e. sandwich and coplanar versus the organic layer. Changes in optical absorption coefficient of 4T-V(CN)(2) based thin films are measured and analyzed to determine the character of the optical transition in the visible range (400-800 nm). Depending on the experimental electrode configuration, magnitude of electroabsorption responses are different, possibly due to different distribution of the externally applied electric field. The results indicate a higher resolution of EA response for the sandwich electrode configuration and confirm the charge transfer exciton character of 4T-V(CN)(2) in contrast to the unsubstituted quaterthiophene 4T. Finally, a third-order nonlinear susceptibility chi((3)) (-omega; omega, 0, 0) of 16 x 10(-12) e.s.u. is obtained. (c) 2006 Elsevier B.V. All rights reserved.