  We present the complete differential decay rates for the process B_s -> J/psi
K^+ K^- including S-wave and P-wave angular momentum states for the K^+ K^-
meson pair. We examine the effect of an S-wave component on the determination
of the CP violating phase 2beta_s. Data from the B-factories indicate that an
S-wave component of about 10% may be expected in the phi(1020) resonance
region. We find that if this contribution is ignored in the analysis it could
cause a bias in the measured value of 2beta_s towards zero of the order of 10%.
When including the K^+K^- S-wave component we observe an increase in the
statistical error on 2beta_s by less than 15%. We also point out the
possibility of measuring the sign of cos2beta_s by using the interference
between the K^+K^- S-wave and P-wave amplitudes to resolve the strong phase
ambiguity. We conclude that the S-wave component can be properly taken into
account in the analysis.
