Recent evidence was found for d-wave symmetry of the superconducting order parame- ter in YBC0. This sets serious theoretical constraints in understanding high Tc superconductivity. A basic question is whether d-wave symmetry is induced from the double CuO2 layer structure of YBC0. Investigation of the superconducting symmetry in other compounds containing a single layer of CuO2 is thus crucial. We propose angularly resolved tunneling studies in single crystals of Hg-1201, Bi-2201 and Tl-1201. All these compounds have a single CuO2 layer and moreover their structure is tetragonal, as opposed to the orthorhombic structure of YBC0. This simplifies much the interpretation of the data. We propose several routes for the growth of single crystals. Tunneling measurements will be done in the Centre de Recherches sur les Trhs Basses Tempiratures in the CNRS in Grenoble. Structural studies will be pursued in collaboration with the Laboratoire de Cristallographie in Grenoble, using conventional and synchrotron X-ray diffraction as well as neutron diffraction.