  In this paper the description of solvable Lie algebras with triangular
nilradicals is extended to Leibniz algebras. It is proven that the matrices of
the left and right operators on elements of Leibniz algebra have upper
triangular forms. We establish that solvable Leibniz algebra of a maximal
possible dimension with a given triangular nilradical is a Lie algebra.
Furthermore, solvable Leibniz algebras with triangular nilradicals of low
dimensions are classified.
