This paper examines the implication of the move to CAPI for data quality by analyzing the conversion from PAPI to CAPI of a subsample of the German Socio-Economic Panel (SOEP) which was done within an experimental design. The 2000 addresses for the sample E of SOEP were split into two subsamples E1 and E2 with the same structure using twin - sample points. Each of the 125 sample points contained 16 addresses (8 for E1 and 8 for E2) and had to be realized in the first wave alternately with PAPI and CAPI mode per interviewer. In the subsequent waves the PAPI mode was partly replaced by CAPI. With this experimental longitudinal design we are able to control for possible interviewer effects in the analysis of mode effects. The paper assesses whether any mode effects are apparent for the response rate. Within the data, we examine monetary dimensions such as gross income, item and unit nonresponse rates. We were able to find some minor effects but our main results show that we have made the shift without introducing strong mode effects.