A new class of very cold young stellar objects (YSOs) has recently been identified and designated "Class 0", which were all undetected by IRAS at 12 microns. These cold YSOs emit the bulk of their luminosity in the submillimeter range and correspond to the youngest protostars known to date. We propose to conduct deep, high angular resolution observations of the envelopes of these cold protostars with ISOCAM at 7.75 and 15 microns, wavelengths where the diffuse interstellar background is relatively intense and extinction at a minimum. This will allow us (1) to check the hypothesis that Class 0 protostars can be seen in absorption at mid-IR wavelengths and are therefore qualitatively different from the more evolved Class I sources detected in near-IR surveys, and (2) to investigate the spatial structure of their circumstellar envelopes with more accuracy than is presently possible in the submillimeter. If achieved, this second objective will set strong constraints on star formation theories, which make definite predictions about the density gradient of protostellar cores/envelopes. In order to supplement this spatial study at far-IR wavelengths (where Class 0 protostars should be detectable in emission with ISO), we also propose super-resolution PHOT observations of one of these sources (not included in the central programme) at 60 and 90 microns.