The equilibrium speciation at 278 K and the dynamics in the systems pH⁺ + qMoO₄²⁻ + rH₂O₂ + sL ⇌ (H⁺)_p(MoO₄²⁻)_q (H₂O₂)_r L_s (L = SO₄²⁻ or Cl⁻) in 0.3 M Na₂(SO₄) and 0.6 M Na(Cl) medium has been determined from ¹⁷O NMR integral and shift data in the range 0.7 ≤ pH ≤ 4.1 and at excess of peroxide ([H₂O₂]_tot/[Mo]_tot = 3). In 0.3 M Na₂(SO₄) medium species with the following compositions were found: MoX₂⁻ (1,1,2,0), MoX₂ (2,1,2,0), MoX₂S²⁻ (2,1,2,1), MoX₂S⁻ (3,1,2,1), Mo₂X₄²⁻ (2,2,4,0), Mo₂X₄⁻ (3,2,4,0) and Mo₂X₆²⁻ (2,2,6,0), where (Mo) corresponds to MoO₄²⁻, (X) to O₂²⁻ or HOO⁻ and (S) to SO₄²⁻. In 0.6 M Na(Cl) medium the diperoxomolybdosulfate complex MoX₂S⁻ is replaced with a novel diperoxomolybdochloride complex MoX₂Cl⁻ (2,1,2,1), where (Cl) corresponds to Cl⁻. The numbers in parentheses refer to the values of p,q,r and s in the formula above. The stoichiometries of each complex are given by the abbreviated formula Mo_qX_rS_s^{(2q + 2s − p)–} and Mo_qX_rCl_s^{(2q + s − p)–}. The following formation constants with 3σ were obtained with values for 0.6 M Na(Cl) in parenthesis: log β_1,1,2,0 = 11.61 (11.61); log β_2,1,2,0 = 13.70 ± 0.10, pK_a = 2.09 (13.86 ± 0.10, pK_a = 2.25); log β_2,1,2,1 = 14.57 ± 0.07 (13.87 ± 0.15 ); log β_3,1,2,1 = 15.27 ± 0.17, pK_a = 0.70, log β_2,2,4,0 = 24.06 ± 0.05 (24.08 ± 0.04); log β_3,2,4,0 = 25.99 ± 0.18, pK_a = 1.93 (26.23 ± 0.17, pK_a = 2.15); log β_2,2,6,0 = 24.02 ± 0.15 (23.9 ± 0.3). The pK_a value for HSO₄⁻ was determined to be 1.06 ± 0.11. At 278 K the MoX₂ and Mo₂X₄ complexes are in chemical exchange and the rate of exchange increases upon protonation. At 312 K there is also measurable exchange between MoX₂S and the MoX₂ and Mo₂X₄ complexes. The dynamics in 0.6 M Na(Cl) medium seem to be very similar to those in the sulfate medium.