Nanocrystals of NaYF4 doped with Yb3+ and Er3+ are synthesized in oleylamine using Y-2(CO3)(3), Yb-2(CO3)(3), Er-2(CO3)(3), Na2CO3, a NH4F as precursors. in contrast to other starting materials normally used for such syntheses, these precursors react even at room temperature to form hexagonal-phase (beta-phase) NaYF4:ErYb nanoparticles. Cubic-phase (alpha-phase) NaYF4:Yb,Er particles are formed only at elevated temperatures (>250 degrees C). The formation of the cubic phase at high temperatures can be suppressed by replacing pure oleylamine with oleic acid/oleylamine mixture. Under optimized reaction conditions, particles with an average particle size of about 7 nm are generated in 84% yield. Heat treatment (30 min, 280 degrees C) of the particles significantly increases the luminescence efficiency. A transparent solution of the heat-treated, nanometer-sized phosphor in toluene shows intense visible light emission upon excitation in the near infrared.