We have studied the high-mass short-period detached eclipsing binary DW Carinae using high-resolution echelle spectroscopy and photoelectric Stroemgren uvby light curves. We derived masses and radii accurate to 1% as well as the effective temperatures and reddening of the system. The high rotational velocities of the component stars meant that blending was a major problem when deriving radial velocities from the spectroscopy. Only helium lines provided good results, and a critique of different methods of measurement showed that spectral disentangling and fitting double Gaussians showed that these were the best able to provide reliable results in this case. The final spectroscopic orbit was calculated using spectral disentangling but we also provided radial velocities by fitting double Gaussians to the He-I 4472-A spectral line in each spectrum as this gives a good idea as to the quality of the data whilst providing a result very close to that adopted to calculate the masses. The light curves of DW Car are published as part of an accompanying paper (Clausen et al., 2006, Cat. <J/A+A/461/1065>).