Effective temperatures (T_eff_) can be determined from (V-J)0, (V-H)0 and (V-K)0 colours that are derived from 2MASS magnitudes. This gives another way to estimate the T_eff_ of faint blue halo stars (V<~15) whose temperatures are now usually deduced from (B_V)0. Transformations (adapted from Carpenter, 2001AJ....121.2851C) are used to change colours derived from the 2MASS data to the Johnson system. T_eff_ is then derived from these colours using an updated Kurucz model. Tables are given to derive T_eff_ as a function of (V-J)0, (V-H)0 and (V-K)0 for a variety of metallicities and suitable for blue horizontal branch and main sequence stars. The temperatures obtained in this way are compared with those in the recent literature for various stars with 5<=V<=15 and T_eff_ in the range 6500 to 9500K; systematic differences are ~100K. An exception is the sample of BHB stars observed by Wilhelm et al. (1999, Cat. ) whose T_eff_ are significantly cooler than those we derive by an amount that increases with increasing temperature. Description:
