Abstract. We consider the Schatten spaces S p in the framework of operator space theory and for any 1 ≤ p = 2 < ∞, we characterize the completely 1-complemented subspaces of S p . They turn out to be the direct sums of spaces of the form S p (H, K), where H, K are Hilbert spaces. This result is related to some previous work of Arazy and Friedman giving a description of all 1-complemented subspaces of S p in terms of the Cartan factors of types 1-4. We use operator space structures on these Cartan factors regarded as subspaces of appropriate noncommutative L p -spaces. Also we show that for any n ≥ 2, there is a triple isomorphism on some Cartan factor of type 4 and of dimension 2n which is not completely isometric, and we investigate L p -versions of such isomorphisms.