An optimised capillary electrophoresis-electrospray mass spectrometric method is presented for the identification and determination of diazepam and its metabolites N'-desmethyldiazepam, oxazepam and temazepam. By investigating constituent parts of the capillary electrophoresis-electrospray mass spectrometric interface and optimising their function, a relatively fast and reproducible method is described for the identification and determination of selected 1,4-benzodiazepines. Optimisation of sheath and auxiliary gas flows, capillary tip tapering, capillary tip positioning, sheath liquid composition and flow rate and pressure application during the separation step have led to acceptable relative standard deviation (RSD) values for migration time and peak area, correlation coefficients and limits of detection. This has been achieved as a result of stabilising the electrospray current prior to analysis, a procedure that takes a matter of minutes when using the method described. Sequential product ion fragmentation (MSn) characterisation of 15 1,4-benzodiazepines is also presented and mechanisms for the observed fragmentation patterns proposed.