We report a facile chemical synthesis of monodisperse colloidal SnSb nanocrystals (NCs) via reaction between Sn NCs and SbCl3 in oleylamine under reducing conditions. In comparison with individual Sn and Sb NCs and their mixtures, we show that through the creation of SnSb alloyed NCs the Li-ion storage properties are enhanced due to combination of high cycling stability of Sb with higher specific Li-ion storage capacity of Sn. In particular, stable capacities of above 700 and 600 mAh g-1 were obtained after 100 cycles of charging/discharging at 0.5C and 4C-rates, respectively (1C corresponding to a current density of 660 mAg-1). Furthermore, Na-ion storage capacities of >350 mAh g-1 and >200 mAh g-1 were obtained at 1C and 20C-rates, respectively. This study highlights the differences between Li- and Na-ion (electro)chemistries and great utility of monodisperse NCs as model systems for understanding size- and compositional effects on the performace of conversion-type electrode materials.