The phase transformations and magnetic properties of melt-spun (Nd 0 . 9 5 La 0 . 0 5 ) 9 . 5 Fe 7 8 - x Co x Cr 2 B 1 0 . 5 (x=0-10) nanocomposites have been investigated. It was found that two magnetic phases, i.e. α-(Fe,Co) and (Nd,La) 2 (Fe,Co) 1 4 B, appeared in all the Co-containing ribbons studied. Co substitution for Fe, i.e. x=2.5-10, increases the Curie temperature (T c ) of both the α-(Fe,Co) and (Nd,La) 2 (Fe,Co) 1 4 B phases at a rate of approximately 20 o C% of Co substitution. Minor grain coarsening can be observed on optimally processed ribbons containing low Co content (x=5). Further increase in Co content was found to have no effect on the average grain size obtained. Instead, an unidentified grain boundary phase was found to surround main phase on ribbons with x=10. This change in microstructure may be one reason that the i H c was preserved at more than 9.5kOe with increasing Co content (up to x=10). Exchange coupling between the magnetically hard and soft phases was found in all samples studied. The remanence, B r , and maximum energy product, (BH) m a x , are improved drastically at x=7.5 and 10, which may arise from the increases in the saturation magnetization of α-(Fe,Co) and (Nd,La) 2 (Fe,Co) 1 4 B as well as the exchange coupling between them. A B r of 10.4kG, i H c of 9.5kOe and (BH) m a x of 19.8MGOe have been achieved in (Nd 0 . 9 5 La 0 . 0 5 ) 9 . 5 Fe 6 8 Co 1 0 Cr 2 B 1 0 . 5 . Moreover, the reversible temperature coefficient of induction (conventionally referred to as α) of optimally processed materials was found to decrease with increasing Co concentration.