problem after only a few Myr, but they must be considered when dealing with the youngest brown dwarfs.

In this paper we investigate spectral signatures of age for substellar objects in pre-main sequence clusters. Previous work by this group and others has established the existence of a population of PMOs in the ONC (@lucas06, hereafter L06; @riddick07). A statistical analysis of the luminosity function by W09 indicates that most substellar objects in the cluster have ages of order 1 Myr, and it is unlikely that the PMO candidates represent a tail of older and more massive objects. However, the size of the planetary mass population cannot be tightly constrained until their ages are known with better precision.

The observations described in this paper were intended to produce the highest resolution spectra yet obtained (R$\approx$5000) of three PMOs in the ONC and a sample of brighter calibrator brown dwarfs with known ages and/or gravities, including three objects on the deuterium-burning threshold in the $\sigma$ Orionis cluster. In the event, we were unable to obtain useful data for two of the three ONC targets. However, the high quality of the data obtained for all of the calibrators has enabled a useful investigation 