This article presents a semigroup approach to the mathematical analysis of the inverse parameter problems of identifying the unknown parameters p(t) and q in the linear parabolic equation u(t)(x, t) = u(xx) + qu(x)(x, t) + p(t)u(x, t), with Dirichlet boundary conditions u(0, t) = psi(0), u(1, t) = psi(1). The main purpose of this paper is to investigate the distinguishability of the input-output mapping Phi[.] : P -> H-1,H- 2 [0, T], via semigroup theory. In this paper, it is shown that if the nullspace of the semigroup T(t) consists of only zero function, then the input-output mapping Phi[.] has the distinguishability property. It is also shown that the types of the boundary conditions and the region on which the problem is defined play an important role in the distinguishability property of the mapping. Moreover, under the light of the measured output data u(x)(0, t) = f(t) the unknown parameter p(t) at (x, t) = (0, 0) and the unknown coefficient q are determined via the input data. Furthermore, it is shown that measured output data f(t) can be determined analytically by an integral representation. Hence the input-output mapping Phi[.] : P -> H-1,H-2 [0, T] is given explicitly interms of the semigroup. (C) 2011 Elsevier Inc. All rights reserved.