In this paper we report a preliminary investigation of laboratory algal cultures and natural populations of marine particles utilizing Direct (i.e. "in-source") Temperature-resolved Mass spectrometry (DT-MS). With this technique the potential exists to characterize microgram quantities of particulate organic matter (POM) at the molecular level. Two particular aspects of the approach set DT-MS apart from previous pyrolysis (Py)-MS investigations. First, temperature resolution allows characterization of both desorption (biolipid) and pyrolysis (biopolymeric) products in a single measurement. Second, an improved sample inlet configuration enables compounds exhibiting a broad range of polarities and molecular weights to be transmitted to the analyzer. The above features, coupled with the rapid analysis time (typically 5 min/sample) and amenability to statistical data reduction procedures, renders DT-MS well suited for profiling or mapping molecular-level variations in POM composition at a spatial and temporal resolution hitherto impractical using conventional biochemical assays. To illustrate the versatility of the approach and extent of (bio)chemical information available from DT-MS, we have analyzed a series of algal cultures together with selected POM samples collected in sediment traps and filtered from hydrocasts. We have also exploited the inherent sensitivity of DT-MS, to obtain information on compositional variability between and, more significantly, within particle size classes. Results of the latter, which was attained by conducting DT-MS measurements on particles sorted by flow cytometry, reveal substantial internal variations in chemical composition.