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The 6 cm formaldehyde (H 2 CO) maser sources in the compact H ii regions NGC 7538 IRS 1 and G29.96À0.02 have been imaged at high resolution ( beam < 50 mas). Using the Very Long Baseline Array and MERLIN, we find the angular sizes of the NGC 7538 masers to be $10 mas (30 AU), corresponding to brightness temperatures $10 8 K. The angular sizes of the G29.96À0.02 masers are $20 mas (130 AU), corresponding to brightness temperatures $10 7 K. Using the VLA, we detect 2 cm formaldehyde absorption from the maser regions. We detect no emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. We find that both NGC 7538 maser components show an increase in intensity on 5-10 yr timescales while the G29.96À0.02 masers show no variability over 2 yr. A search for polarization provides 3 upper limits of 1% circularly polarized and 10% linearly polarized emission in NGC 7538 and of 15% circularly polarized emission in G29.96À0.02. A pronounced velocity gradient of 28 km s À1 arcsec À1 (1900 km s À1 pc À1 ) is detected in the NGC 7538 maser gas.

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