The paper presents a theoretical treatment of on-line identification issues and develops parameter identification algorithms for a class of single input single output (SISO) linear time delay systems (LTDS) with uncertain time-invariant parameters. The delays can also be unknown if their possible values belong to a set consisting of a finite number of known values. Since only output measurements are assumed to be available, a sliding mode-based observer of an underlying SISO LTDS with uncertain parameters is constructed and utilized to design an adaptive identifier of system parameters. The theory is supported by an application of the developed algorithms to transient fuel identification in a port fuel injected (PFI) internal combustion engine. (c) 2007 Elsevier Ltd. All rights reserved.