This article investigates studio practice and replication in Domenichino's studio. It is focused on the recently discovered Cumaean Sibyl, formerly in the Ratta collection, which is presented here as an autograph second version of the Cumaean Sibyl in the Pinacoteca Capitolina, Rome. The interdependence of these two versions has been established both through direct confrontation and through investigation of specially commissioned X-rays. Further X-rays of Domenichino's Persian Sibyl in the Wallace Collection indicate that both versions of the Cumaean Sibyl derive directly from its design. The contours and principal features of the heads in all three canvases correspond so perfectly that some means of transfer in Domenichino's studio must have been involved.