The electrocatalytic oxidation of formaldehyde on Pt(111) under potentiostatic conditions and periodic potential oscillations was studied by using optical second harmonic generation (SHG). Under potentiostatic conditions, order-disorder and order-order phase transitions were observed. Under galvanostatic conditions, oscillations of the SHG field occurred, corresponding to the potential oscillations. However, in the dynamic regime only the evidence of a structural order-disorder phase transition is present in SHG data, This might be due to a much higher time constant of the surface ordering in comparison to the oscillation period.