The electronic structures of N,1,3-tris(1,1-dimethylethyl)-cyclodisilazan-2-amine (I) and 2,3,5,5-tetrakis(trimethylsilyl) cyclopentadiene (II) have been investigated by HeI and HeII UV photoelectron spectroscopy (UPS) and quantum chemical calculations. We discuss the influence of substituent effects on their electronic structure and thermodynamic stability. Our study shows that trimethylsilyl substituents have strong influence on the electronic structure of cyclopentadiene via inductive effect. Their influence on thermodynamic stability is also pronounced. In substituted cyclodisilazanes hyperconjugative influence of alkylsilyl groups was shown to cause relative thermodynamic stabilization of the cyclodisilazane system. (C) 2010 Elsevier B. V. All rights reserved.