Many civil and mechanical engineering structures exhibit nonlinear hysteretic behavior when subject to dynamic loads, such as earthquakes. The modeling and identification of non-linear hysteretic systems with stiffness and strength degradations is a practical but challenging problem encountered in the engineering field. A recently developed technique, referred to as the adaptive quadratic sum-square error with unknown inputs (AQSSE-UI), is capable of identifying time dependant parameters of nonlinear hysteretic structures. In this paper, the AQSSE-UI technique is applied to the parametric identification of nonlinear hysteretic reinforced concrete structures with stiffness and strength degradations, and the performance of the AQSSE technique is demonstrated by the experimental test data. A 1/3 scaled 2-story RC frame has been tested experimentally on the shake table at NCREE, Taiwan. This 2-story RC frame was subject to different levels of ground excitations back to back. The structure is firstly considered as an equivalent linear model with time-varying stiffness parameters, and the tracking of the degradation of the stiffness parameters is carried out using the AQSSE-UI technique. Then the same RC frame is considered as a nonlinear hysteretic model with inelastic hinges following the generalized Bouc-Wen model, and the time-varying nonlinear parameters are identified again using the AQSSE-UI technique. Experimental results demonstrate that the AQSSE technique is quite effective for the tracking of: (i) the stiffness degradation of linear structures, and (ii) the non-linear hysteretic parameters with stiffness and strength degradations.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.