We consider K-interpolation spaces involving slowly varying functions, and derive necessary sufficient conditions for a Holmstedt-type formula to be held in the limiting case θ 0 = 1 ∈ { , } $\theta _0=\theta _1\in \lbrace 0,1\rbrace$ . also study ( ) (0,1)$ Applications are given Lorentz–Karamata spaces, generalized gamma Besov spaces.