We have investigated, experimentally and theoretically, the series (Na1-xLix)2IrO3. Contrary to what has been believed so far, only for x≤0.25 does the system form uniform solid solutions where Li preferentially goes to the Ir2Na planes, as observed in our density functional theory calculations and consistent with x-ray diffraction analysis. For larger Li content, as evidenced by powder x-ray diffraction, scanning electron microscopy, and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na3LiIr2O6 phase with alternating Na3 and LiIr2O6 planes, and a Li-rich phase close to pure Li2IrO3. For x≤0.25 we observe (1) an increase of c/a with Li doping up to x=0.25, despite the fact that c/a in pure Li2IrO3 is smaller than in Na2IrO3, and (2) a gradual reduction of the antiferromagnetic ordering temperature TN and ordered moment. In view of our results showing clear evidence for phase separation for 0.25≤x≤0.6, more detailed studies are needed to confirm the presence or absence of phase separation at the higher doping x∼0.7, where a continuum quantum phase transition has been proposed previously. © 2014 American Physical Society.