For plant wide modelling of wastewater treatment, it is necessary to develop a suitable state variables interface for integrating state of the art models of ASM and ADM1. ADM1 currently describes such an interface, however, its suitability needs to be experimentally evaluated. In this study, we characterised activated sludge under aerobic and anaerobic conditions to obtain representative state variables for both models. ASM state variables of XS, XH and XI (as obtained from aerobic tests) and ADM1 state variables of XC and XI (as obtained from anaerobic tests) were then correlated to assess the suitability of current interface. Based on the seven datasets of this study and seven datasets from literatures, it was found that in general ASM state variables were well correlated to the state variables of ADM1. The ADM1 state variable of XC could be correlated to the sum of state variables of XS and XH, while XI in both the models showed direct correspondence. It was also observed that the degradation kinetics of XC under anaerobic condition could be better described by individual degradation kinetics of XS and XH. Therefore, to establish a one to one correspondence between ASM and ADM1 state variables and better description of degradation kinetics in ADM1, replacing the composite variable of XC by the state variables of XS and XH is recommended.