The magnetic properties and crystal structures of $Sm_yGd_{2-y}Fe_{17-x}Si_x$ alloys ($0leq;xleq2;and;y=0~1.67$) have been investigated using x-ray diffraction and magnetic measurements. The $Sm_yGd_{2-y}Fe_{17-x}Si_x$ specimens were crystallized to the rhombohedral $Th_2Zn_{17}-structure$ with less than $5mol\%$ of impurities. The unit cells of the mixed rare-earth samples are smaller than those of $Sm_2Fe_{17};and;Gd_2Fe_{17}.$ For example, the $T_c;of;SmGdFe_{17};(255^{circ}C)$ is approximately 160 and $800^{circ}C)$ higher than that of $Sm_2Fe_{17};and;Gd_2Fe_{17},$ respectively. The $T_cs$ measured for $Sm_yGd_{2-y}Fe_{17-x}Si_x$ samples, 280 to $290^{circ}C)$, are among the highest values observed for a $R_2Fe_{17-x}M_x$ intermetallic where M is a substituent other than cobalt.