Studies on 4-nitropyridine 1-oxide (I) and related N-oxides (II, N): Irradiation yields I exclusively, which undergoes further photoisomerization. At higher II concentration (ca. 10⁻²M), both dark and light reactions form K, supporting a second-order mechanism for K formation dependent on II concentration. The photochemical process from I to II is first-order to I but very slow, confirmed by comparative experiments. 4-Nitrosopyridine 1-oxide (N) is reduced to II via light reaction with slow rate and no concentration effect (10⁻⁵~10⁻²M), excluding N as an intermediate in I-to-II photochemical reactions. Current mechanisms lack direct proof, requiring more experimental data to understand the complex photolysis mechanisms of I and related N-oxides. Studies on Leucomycin A₁: Leucomycins consist of six components (A₁, A₂, B₁, B₂, B₃, B₄), with A₁ as the major macrolide antibiotic (4-O-(4-O-isovalerylmycaropyranosyl)mycaminopyranoside of a large-membered lactone). Leucomycin A₁ is similar to the new component A₃ in chemical/physicochemical properties. Converting A₁ to triacetate and A₃ to diacetate, their NMR (100 Mc), IR spectra, and thin-layer chromatography behavior are identical. Mixed melting point of A₁ triacetate and A₃ diacetate confirms the structure: Leucomycin A₁ is Leucomycin A₃ with one acetyl group eliminated (Ia).