The silicon sites tetrahedrally connected to aluminum in framework positions of a molecular sieve may be identified by a selective reintroduction of the hetero-nuclear Al-27-Si-29 dipolar interaction through Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) NMR, In this rotor synchronized Si-29 MAS experiment, an effective dipolar dephasing of the Si-O-Al, over Si-O-Si, environments is shown to aid in the identification of silicon sites in the immediate vicinity of aluminum. Application of the method in the structurally interesting and novel molecular sieve ETAS-10 provides valuable insights on the details of aluminum substitution in the zeolite lattice and further leads to the first direct NMR estimate of Al-Si distance (r(Al-Si) = 323 +/- 5 pm) in ETAS-10. (C) 2004 Elsevier B.V. All rights reserved.