Lightcurve analysis gave a rotation period of 12.19 and 12.1923 hours with a brightness amplitude of 0.23 and 0.22 magnitude, respectively (U=2+/3-).

Modelling gave a concurring period of 12.18978 hours, as well as two spin axis of (19.0°, 44°) and (250.0°, 64°) in ecliptic coordinates (λ, β).

[13] According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Postrema measures between 34.696 and 47.00 kilometers in diameter and its surface has a notably low albedo between 0.0137 and 0.05.

[4][5][6][7][8][9][10][12] The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0409 and a diameter of 43.42 kilometers based on an absolute magnitude of 10.9.

These would be the asteroids 4420 Alandreev (highest numbered) and 2536 Kozyrev, officially discovered on 15 August 1939, more than a year after Postrema.