  We present measurements on hybrid single-electron turnstiles with
superconducting leads contacting a normal island (SINIS). We observe Andreev
tunneling of electrons influencing the current plateau characteristics of the
turnstiles under radio-frequency pumping. The data is well accounted for by
numerical simulations. We verify the dependence of the Andreev tunneling rate
on the turnstile's charging energy. Increasing the charging energy effectively
suppresses the Andreev current.
