UV/Visible absorption spectroscopy, coupled with multiparameter least-squares analysis, was used to determine profiles of gas-phase temperature and NO concentration in the dark-zone region of two RDX-based solid propellants (XM39 and M43) at pressure up to 3.55 MPa. At low pressures (e.g., P < 1.6 MPa for M43 and P < 2.17 MPa for XM39), a uniform dark zone was observed between the burning surface and the luminous flame, with intermittent flamelet attachment to the burning surface. As pressure increased, flamelet attachment became very pronounced, and the uniform dark zone was rarely seen. Temperature and NO concentration profiles in the dark zone of both propellants were very uniform when a uniform dark zone existed. Under this situation, M43 and XM39 were found to have similar dark-zone temperatures (around 1050 K for M43 and 1180 K for XM39); however, M43 has about 5% more NO mole fraction than XM39 in the dark-zone region (20% vs 15%). On the other hand, when luminous flamelets were attached to the burning surface, line-of-sight-averaged NO concentrations (where a fixed path length is used in data analysis) exhibited a rapid reduction with increased distance from the burning surface. This reduction results from the shorter intercepted lengthy by the UV light beam in the dark zone as the vertical distance from the burning surface increases; the deduced NO concentration is thereby decreased if the whole propellant diameter is used as the path length in data analysis. According to video records, M43 has a smaller dark zone than XM39 at the same pressure, and its dark zone decreases more rapidly with an increase of pressure than XM39 in the pressure range studied. M43 was found to be able to maintain a stable ＇＇flame-burning＇＇ mode combustion at a lower pressure than XM39.