摘要： We present new Keck/NIRC2 3–5 μm infrared photometry of the planetary-mass companion to ROXS 42B in L(cid:48), and for the ?rst time in Brackett-α (Brα) and in Ms-band. We combine our data with existing near-infrared photometry and K-band (2–2.4 μm) spectroscopy and compare these data with models and other directly imaged planetary-mass objects using forward modeling and retrieval methods in order to characterize the atmosphere of ROXS 42B b. The ROXS 42B b 1.25–5 μm spectral energy distribution most closely resembles that of GSC 06214 B and κ And b, although it has a slightly bluer Ks?Ms color than GSC 06214 B and thus currently lacks evidence of a circumplanetary disk. We cannot formally exclude the possibility that any of the tested dust-free/dusty/cloudy forward models describe the atmosphere of ROXS 42B b well. However, models with substantial atmospheric dust/clouds yield temperatures and gravities that are consistent when ?t to photometry and spectra separately, whereas dust-free model ?ts to photometry predict temperatures/gravities inconsistent with the ROXS 42B b K-band spectrum and vice-versa. Atmospheric retrieval on the 1–5 μm photometry places a limit on the fractional number density of CO2 of log(nCO2 ) < ?2.7, but provides no other constraints so far. We conclude that ROXS 42B b has mid-IR photometric features that are systematically di?erent from other previously observed planetary-mass and ?eld objects of similar temperature. It remains unclear whether this is in the range of the natural diversity of targets at the very young (～2 Myr) age of ROXS 42B b or unique to its early evolution and environment.