The concentration and chemical composition of preservative or poison to use in sediment trap studies continue to present an important unresolved question. Past laboratory/field experiments designed to answer this question are difficult to interpret, because so-called analogs have been used instead of actual trap materials, which are compositionally complex. This paper presents our results on the in situ effects of formalin, azide and mercuric ion on material collected in MULTITRAPS set at 100 and 300 m for a period of six days in a coastal environment, and at 150 m for a period of 20.6 days in an oligotrophic environment. Effective preservative/poison concentrations used were predetermined from laboratory tests. Parameters tested for relative effects included particulate retention of C, N and selected trace metals, and the effects of the various preservatives/poisons introduced via diffusion chambers or free in solution on in situ microbial growth. In addition, the potential contaminating role of large, nonsinking zooplankton (i.e., “swimmers”) was investigated. Substantial differences between treatments were observed. Effects were not uniform, and appeared to be parameter-specific. For example, during the short-term deployment (six days), no significant differences in C flux were observed at 100 m, regardless of preservative used. Traps treated with azide yielded significantly lower N values. At 300 m (short-term deployment), and 150 m (long-term deployment), the azide treatments produced the lowest mean C and N values. Conversely, the formalin traps gave the highest C and N values relative to all treatments at these depths. In terms of metals, >70% of both Cd and Mn were lost to the trap solutions, regardless of oceanic area or time deployed, while most of the Fe tended to remain in the particulate phase. Zinc, largely in association with the particulate phase over the six-day deployment, was lost to the density solution during the 20.6-day deployment, while Pb results were intermediate between these extremes. Results of the diffusion chamber experiment indicate that the formalin and mercuric ion treatments were equally effective regardless of the mode of introduction (i.e., diffusion chamber or free in solution). Azide did not appear as effective when introduced via diffusion.