  We point out that for solar neutrino oscillations with the mass-squared
difference of Delta m^2 ~ 10^-10 - 10^-9 eV^2, traditionally known as ``vacuum
oscillation'' range, the solar matter effects are non-negligible, particularly
for the low energy pp neutrinos. One consequence of this is that the values of
the mixing angle theta and pi/2-theta are not equivalent, leading to the need
to consider the entire physical range of the mixing angle 0<=theta<=pi/2 when
determining the allowed values of the neutrino oscillation parameters.
