Keywords:Mixed boundary condition Zaremba problem Resolvent difference Dirichlet-to-Neumann operator Krein resolvent formula Spectral asymptotics Weak Schatten class Nonstandard pseudodifferential operator For a second-order symmetric strongly elliptic operator A on a smooth bounded open set in R n , the mixed problem is defined by a Neumann-type condition on a part Σ + of the boundary and a Dirichlet condition on the other part Σ − . We show a Kreȋn resolvent formula, where the difference between its resolvent and the Dirichlet resolvent is expressed in terms of operators acting on Sobolev spaces over Σ + . This is used to obtain a new Weyltype spectral asymptotics formula for the resolvent difference (where upper estimates were known before), namely s j j 2/(n−1) → C 2/(n−1) 0,+ , where C 0,+ is proportional to the area of Σ + , in the case where A is principally equal to the Laplacian.The mixed boundary value problem for a second-order strongly elliptic symmetric operator A on a smooth bounded open set Ω ⊂ R n with boundary Σ , in case of the Laplacian also called the Zaremba problem, is defined by a Neumann-type condition on a part of the boundary Σ + and a Dirichlet condition on the other part Σ − . It does not have the regularity of standard elliptic boundary problems (the L 2 -domain is at best in H 3 2 −ε (Ω)). It has been analyzed with regards to regularity and mapping properties e.g. in Peetre [45,46], Shamir [52], Eskin [14], Pryde [48], Rempel and Schulze [49], Simanca [53], Harutyunyan and Schulze [33]. We shall here study it from the point of view of extension theory for elliptic operators. There has been a recent revival in the interest for connections between abstract extension theories for operators in Hilbert space (as initiated by Krein [36], Vishik [54], Birman [6], Grubb [22] and others) and interpretations to boundary value problems for partial differential operators. Cf. e.g. Amrein and Pearson [3], Pankrashkin [44], Behrndt and Langer [4], Ryzhov [50], Brown, Marletta, Naboko and Wood [13], Alpay and Behrndt [2], Malamud [41], based on boundary triples theory (as developed from the book of Gorbachuk and Gorbachuk [20] and its sources). Other methods are used in the works of Brown, Grubb and Wood [12,28], Posilicano and Raimondi [47], Gesztesy and Mitrea [16-18] (and their references); see also and Abels, Grubb and Wood [1]. One of the interesting aims has been to derive Kreȋn resolvent formulas that link the resolvent of a general operator with the resolvent of a fixed reference operator by expressing the difference in terms of operators connected to the boundary.For the mixed problem, a Kreȋn resolvent formula connecting the operator to the Dirichlet realization was worked out in [44], based on boundary triples theory. A different formula results from [22,24], see also [12], Sect. 3.2.5. Observations on the connection with the Neumann realization were given in [41]. An upper bound for the spectral behavior of the resolvent difference was shown by Birman in [6].In the present paper we sha...